xlib_libXdmcp: add missing dependency on host-pkgconf

xlib_libXdmcp: add missing dependency on host-pkgconf

The libXdmcp configure script uses pkg-config, but the Buildroot
package does not depend on host-pkgconf. This is not seen by the
autobuilders most likely because another package that is always built
before libXdmcp builds pkg-config.

However, running:

$ make xlib_libXdmcp

triggers the following build failure:

checking pkg-config is at least version 0.9.0... ./configure: line 12323: /home/thomas/projets/buildroot/output/host/bin/pkg-config: No such file or directory
no
[...]
checking for XDMCP... configure: error: in `/home/thomas/projets/buildroot/output/build/xlib_libXdmcp-1.1.2':
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.

This was detected using per-package target/host folders, because with
this, only the dependencies explicitly expressed by a package are
available to the package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>