[Xcb] check for xproto

[Jeremy C. Reed]

configure.ac for libxcb should also check for xproto with 
PKG_CHECK_MODULES.

Building libxcb uses Xauth.h which uses Xfuncproto.h and Xfuncs.h.

Or it should depend on a xau or xdmcp that depend on xproto via their 
pkg-config files.