XQuartz: Include os.h for OsAbort()

Fixes regression from 5b9a52be7e

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>

hw/xquartz/GL/capabilities.c

@@ -31,6 +31,10 @@
 
 #include "capabilities.h"
 
+#define Cursor X_Cursor
+#include "os.h"
+#undef Cursor
+
 static void handleBufferModes(struct glCapabilitiesConfig *c, GLint bufferModes) {
     if(bufferModes & kCGLStereoscopicBit) {
 	c->stereo = true;