PtexPlatform.h: fix case-sensitive build issues on MinGW

Closes: #85

Author: davvid

src/ptex/PtexPlatform.h

@@ -60,7 +60,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 #define NOMINMAX 1
 
 // windows - defined for both Win32 and Win64
-#include <Windows.h>
+#include <windows.h>
 #include <malloc.h>
 #include <io.h>
 #include <tchar.h>