Add parenthesis around some generated code so it compiles.  This new
code only shows up when the X11 dissector is updated from the latest
xcbproto & mesa, which I will commit real soon now (RSN).

Change-Id: Ic750df3b4406f107fd850ade891ee093054c8d6d
Reviewed-on: https://code.wireshark.org/review/5882
Reviewed-by: Stephen Fisher <sfisher@sdf.org>