Fix dependencies for TrapProto

The .pc file of TrapProto says that it depends on xt, even through it
doesn't depend on it at compile time. However, if xt isn't present
when TrapProto has been installed, the compilation of Xserver fails
telling that there is a missing dependency in TrapProto on Xt.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>