Fixed warning.

include/bx/platform.h

@@ -122,7 +122,9 @@
 #	define BX_PLATFORM_XBOX360 1
 #elif defined(_WIN32) || defined(_WIN64)
 // http://msdn.microsoft.com/en-us/library/6sehtctf.aspx
-#	define NOMINMAX
+#	ifndef NOMINMAX
+#		define NOMINMAX
+#	endif // NOMINMAX
 #	include <windows.h>
 #	if !defined(WINAPI_FAMILY) || (WINAPI_FAMILY == WINAPI_FAMILY_DESKTOP_APP)
 #		undef  BX_PLATFORM_WINDOWS