Fix GLFW_INCLUDE_GLEXT being ignored for glcorearb

The GLFW_INCLUDE_GLCOREARB branch was left out when GLFW_INCLUDE_GLEXT
was originally added, for reasons that are lost to history.  The current
versions of these headers seem to co-exist just fine.

Issue reported on IRC.

(cherry picked from commit 309d79376f)

include/GLFW/glfw3.h

@@ -190,6 +190,9 @@ extern "C" {
  #else /*__APPLE__*/
 
   #include <GL/glcorearb.h>
+  #if defined(GLFW_INCLUDE_GLEXT)
+   #include <GL/glext.h>
+  #endif
 
  #endif /*__APPLE__*/