Put back the include of windows.h so we are back to version 17 of the file.

include/GL/glext.h

@@ -34,6 +34,11 @@ extern "C" {
 ** version 1.2.1 Specification.
 */
 
+#if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__)
+#define WIN32_LEAN_AND_MEAN 1
+#include <windows.h>
+#endif
+
 #ifndef APIENTRY
 #define APIENTRY
 #endif