Xcb: Fix compilation with -no-c++11

nullptr (introduced in commit d7db6c6c

) is a C++11 keyword.

Change-Id: Id6aa639f5d2ddbe8b228f8648754c2b24ef53799
Reviewed-by: David Rosca <nowrep@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>