xcb-util: add dependency on libxcb

The xcb-util lacks a dependency on libxcb. Without this dependency, it
fails to build with:

checking for XCB... no
configure: error: in `/home/thomas/projets/buildroot/output/build/xcb-util-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.

Alternatively, you may set the environment variables XCB_CFLAGS
and XCB_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>