Fixed issue with filename case sensitivity.

If I try to build with xbuild in my Arch linux, I get following error:

```
~/dev/SharpFont> xbuild

----build stuff scissored out----

Build FAILED.
Errors:

/home/lazzu/dev/SharpFont/SharpFont.sln (default targets) ->
(Build target) ->
/home/lazzu/dev/SharpFont/SharpFont/SharpFont.csproj (default targets) ->
/usr/lib/mono/4.0/Microsoft.CSharp.targets (CoreCompile target) ->

	: error CS2001: Source file `TrueType/EncodingId.cs' could not be found
	: error CS2001: Source file `TrueType/Pclt.cs' could not be found
	: error CS2001: Source file `TrueType/PlatformId.cs' could not be found

	 0 Warning(s)
	 3 Error(s)

Time Elapsed 00:00:00.5918730
```

The reason is in case sensitivity. I have fixed that in this commit.