Add GL_CORE_WINDOWS define to glcore.h

This is for consistency with glproto. The GL_CORE defines should probably be
dropped.

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Author: Alexander Gottwald <alexander.gottwald@s1999.tu-chemnitz.de>
Date:   Mon Jun 21 13:35:05 2004 +0000

    Bug 782: Merge native OpenGL for Windows from CYGWIN branch

include/GL/internal/glcore.h

@@ -41,6 +41,7 @@
 #define GL_CORE_SGI  1
 #define GL_CORE_MESA 2
 #define GL_CORE_APPLE 4
+#define GL_CORE_WINDOWS 8
 
 typedef struct __GLcontextRec __GLcontext;