https://github.com/mono/mono/pull/5680 added mmap/sendfile prototypes
but we should only use those when ANDROID_UNIFIED_HEADERS is defined
to avoid getting errors like:
```
/Users/alexander/dev/xamarin-android/external/mono/mono/utils/mono-compiler.h:157:7: error: conflicting types for 'mmap'
void* mmap (void*, size_t, int, int, int, __mono_off32_t);
^
/Users/alexander/android-toolchain/toolchains/arm-linux-androideabi-clang/bin/../sysroot/usr/include/sys/mman.h:47:15: note: previous declaration is here
extern void* mmap(void *, size_t, int, int, int, off_t);
^
/Users/alexander/dev/xamarin-android/external/mono/mono/utils/mono-compiler.h:166:9: error: conflicting types for 'sendfile'
ssize_t sendfile (int out_fd, int in_fd, __mono_off32_t* offset, size_t count);
^
/Users/alexander/android-toolchain/toolchains/arm-linux-androideabi-clang/bin/../sysroot/usr/include/sys/sendfile.h:36:16: note: previous declaration is here
extern ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
```
#endif
/* Used when building with Android NDK's unified headers */
-#if defined(HOST_ANDROID)
+#if defined(HOST_ANDROID) && defined (ANDROID_UNIFIED_HEADERS)
#if __ANDROID_API__ < 21
typedef int32_t __mono_off32_t;
#endif /* !sendfile */
#endif /* __ANDROID_API__ < 21 */
-#endif /* HOST_ANDROID */
+#endif /* HOST_ANDROID && ANDROID_UNIFIED_HEADERS */
#endif /* __UTILS_MONO_COMPILER_H__*/