cups: add host-pkgconf as a dependency

cups: add host-pkgconf as a dependency

cups configure script checks for a pkg-config program. If host-pkgconf
has not been built prior to cups, it uses the system pkg-config, which
might potentially lead to bogus results:

checking for i586-buildroot-linux-musl-pkg-config... no
checking for pkg-config... /usr/bin/pkg-config

With this patch applied:

checking for i586-buildroot-linux-musl-pkg-config... no
checking for pkg-config... /home/thomas/projets/buildroot/output/host/usr/bin/pkg-config

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>