hw/xwin: Add an idempotency guard to glwindows.h

Add an idempotency guard to glwindows.h

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/glx/glwindows.h

@@ -28,6 +28,9 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#ifndef GLWINDOWS_H
+#define GLWINDOWS_H
+
 #include <GL/gl.h>
 
 typedef struct {
@@ -53,3 +56,5 @@ int glWinSelectImplementation(int native);
 #define GLWIN_TRACE_MSG(a, ...)
 #define GLWIN_DEBUG_MSG(a, ...)
 #endif
+
+#endif