Haiku: fix the glew.c hack

drivers/gl_context/glew.c

@@ -1,4 +1,8 @@
-#ifndef GLEW_ENABLED
+#ifdef __HAIKU__
+	#undef GLEW_ENABLED
+#endif
+
+#ifdef GLEW_ENABLED
 /*
 ** The OpenGL Extension Wrangler Library
 ** Copyright (C) 2002-2008, Milan Ikits <milan ikits[]ieee org>