xlib_libX11: fix compilation of makekeys

xlib_libX11 builds a makekeys executable for the host, but uses the
targets X11_CFLAGS. This leads to build failures like
http://autobuild.buildroot.net/results/411eb3aefea859a7e31986a44acd50b475f174cb/



This problem was introduced by the version bump, because then also
the AUTORECONF was removed so the existing fix didn't work anymore.

As a slightly cleaner solution, just remove X11_CFLAGS from the
Makefile.  We know we don't need it, because the X11 stuff is in
$(HOST_DIR)/usr/include which is already in HOST_CFLAGS.

[Peter: reword comments]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>