Avoid error/warning about implicit function declaration

Function xorgGlxCreateVendor() is defined in glx_extinit.h, if this header is
not included, we might get either error or warning. This header also need to
be included after scrninststr.h header as it defines some structures used
in glx_extinit.h

(cherry picked from commit 8c6c584377)

unix/xserver/hw/vnc/xvnc.c

@@ -47,6 +47,9 @@ from the X Consortium.
 #include <X11/Xproto.h>
 #include <X11/Xos.h>
 #include "scrnintstr.h"
+#if XORG >= 120
+#include "glx_extinit.h"
+#endif
 #include "servermd.h"
 #include "fb.h"
 #include "mi.h"