add libdir/exec_prefix to xcb-xproto.pc
add libdir/exec_prefix to xcb-xproto.pc

If we want to base other paths off of exec_prefix or libdir (like the
python path), we can't because the xcb-proto.pc.in file omits those
variables.  Add them so people can leverage them in their own builds.

Signed-off-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@gentoo.org>
Signed-off-by: Julien Danjou <5c682c2d1ec4073e277f9ba9f4bdf07e5794dabe@danjou.info>

stapelberg/xcb-proto,nbaum/xcb-proto,stapelberg/xcb-proto,nbaum/xcb-proto