xlib_libFS: Add missing host-pkgconf dependency

Fixes compile error:

checking for FS... configure: error: in `/home/br/br/output/build/xlib_libFS-1.0.6':
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 FS_CFLAGS
and FS_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

To get pkg-config, see <http://pkg-config.freedesktop.org/

>.
See `config.log' for more details

using this minimal defconfig

BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XAPP_XFS=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>