Windows.h -> windows.h for cross-mingw build

src/filesys.cpp

@@ -28,7 +28,7 @@ namespace fs
 #ifdef _WIN32 // WINDOWS
 
 #define _WIN32_WINNT 0x0501
-#include <Windows.h>
+#include <windows.h>
 #include <stdio.h>
 #include <malloc.h>
 #include <tchar.h>