x11r7/xcb-util-keysyms: depends on host-pkgconf and libxcb

Fix build failure:
./configure: line 11453: /home/tetsuya/buildroot/br/output/host/usr/bin/pkg-config: No such file or directory
configure: error: in `/home/tetsuya/buildroot/br/output/build/xcb-util-keysyms-0.3.9':
configure: error: The pkg-config script could not be found or is too old.  Make sure it
is in your PATH or set the PKG_CONFIG environment variable to the full
path to pkg-config.

Fix build failure:
configure: error: Package requirements (xcb >= 1.4) were not met:

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>