Fixes windows headers to use the exact mingw capitalization.

This is necessary for cross-compilation on linux, as linux treats different capitalization as different files.

Core/Contents/Include/PolyWinCore.h

@@ -31,9 +31,9 @@
 #include <winsock2.h>
 #include <windows.h>
 #include <windowsx.h>
-#include <WinUser.h>
+#include <winuser.h>
 
-#include <MMSystem.h>
+#include <mmsystem.h>
 #include <regstr.h>
 
 

Core/Contents/Source/OSBasics.cpp

@@ -23,7 +23,7 @@
 #include "OSBasics.h"
 #ifdef _WINDOWS
 	#include <windows.h>
-	#include <Shellapi.h>
+	#include <shellapi.h>
 #else
 	#include <dirent.h>
 	#include <sys/types.h>

Core/Contents/Source/PolyWinCore.cpp

@@ -34,9 +34,9 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-#include <Shlobj.h>
-#include <Shellapi.h>
-#include <Commdlg.h>
+#include <shlobj.h>
+#include <shellapi.h>
+#include <commdlg.h>
 
 #if defined(_MINGW)
 #ifndef MAPVK_VSC_TO_VK_EX