Disable -Werror=format-security on GCC as the input is already validated by the caller

Disable -Werror=format-security on GCC as the input is already validated by the caller

Jonathan Pryor (jonpryor) commented on f49164b:

The message parameter is validated in managed code:
https://github.com/mono/mono/blob/master/mcs/class/Mono.Posix/Mono.Unix.Native/Syscall.cs#L3200
https://github.com/mono/mono/blob/master/mcs/class/Mono.Posix/Mono.Unix/UnixMarshal.cs#L366

This change means that '%m' isn't useful in format strings to Syscall.syslog(),
as it'll be escaped instead of "passed through" unchanged.