Fix a recursive lock in libX11.

The XIfEvent() predicate function is not allowed to take the X
display lock, because it is already taken in XIfEvent() itself.

With libX11 1.8.1 this causes a dead-lock because libX11 now
unconditionnally calls XInitThreads(), making the X display lock
always active.

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>