[System.Xml] Fix casing of System.dll reference in Xslt compiler

[System.Xml] Fix casing of System.dll reference in Xslt compiler

Some XML tests enabled as part of 93e845de160cb40830c74f5d0fd7fa82fe2f1d35 failed
on Linux because of the lowercased system.dll reference in referencesource:

```
1) CompileNoLineInfoSource (MonoTests.System.Xml.Xsl.MsxslScriptTests.CompileNoLineInfoSource)
   System.Xml.Xsl.XsltException : Script compile errors:
error CS0006: Metadata file `system.dll' could not be found

  at System.Xml.Xsl.XsltOld.Compiler.CompileAssembly (System.Xml.Xsl.XsltOld.ScriptingLanguage lang, System.Collections.Hashtable typeDecls, System.String nsName, System.Security.Policy.Evidence evidence) [0x00229] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Data.SqlXml/System/Xml/Xsl/XsltOld/Compiler.cs:823
  at System.Xml.Xsl.XsltOld.Compiler.CompileScript (System.Security.Policy.Evidence evidence) [0x00016] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Data.SqlXml/System/Xml/Xsl/XsltOld/Compiler.cs:749
  at System.Xml.Xsl.XsltOld.Compiler.Compile (System.Xml.Xsl.XsltOld.NavigatorInput input, System.Xml.XmlResolver xmlResolver, System.Security.Policy.Evidence evidence) [0x0009d] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Data.SqlXml/System/Xml/Xsl/XsltOld/Compiler.cs:200
  at System.Xml.Xsl.XslTransform.Compile (System.Xml.XPath.XPathNavigator stylesheet, System.Xml.XmlResolver resolver, System.Security.Policy.Evidence evidence) [0x00022] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Xml/System/Xml/Xslt/XslTransform.cs:264
  at System.Xml.Xsl.XslTransform.Load (System.Xml.XPath.XPathNavigator stylesheet, System.Xml.XmlResolver resolver) [0x0000e] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Xml/System/Xml/Xslt/XslTransform.cs:82
  at System.Xml.Xsl.XslTransform.Load (System.Xml.XPath.IXPathNavigable stylesheet, System.Xml.XmlResolver resolver) [0x0000e] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Xml/System/Xml/Xslt/XslTransform.cs:69
  at System.Xml.Xsl.XslTransform.Load (System.Xml.XPath.IXPathNavigable stylesheet) [0x00000] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/referencesource/System.Xml/System/Xml/Xslt/XslTransform.cs:63
  at MonoTests.System.Xml.Xsl.MsxslScriptTests.CompileNoLineInfoSource () [0x00018] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/System.XML/Test/System.Xml.Xsl/MsxslScriptTests.cs:210
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-i386/mcs/class/corlib/System.Reflection/MonoMethod.cs:305
```