Win32: we need GLew.h before GL.h with the headers we are using

This is probably not perfect, but our contribs header need that

modules/video_output/opengl.h

@@ -53,6 +53,9 @@
 # elif USE_OPENGL_ES == 1
 #  include <GLES/gl.h>
 # else
+#  ifdef WIN32
+#   include <GL/glew.h>
+#  endif
 #  include <GL/gl.h>
 # endif
 #endif