opkg: fix configure step by adding host-pkgconf dependency

The opkg configure.ac uses the PKG_CHECK_MODULES() m4 macro, which is
provided by the .m4 files installed by host-pkgconf, so we must have
this package as a dependency.

Fixes:

  http://autobuild.buildroot.org/results/069fcab6e7dd5a1f07251ba6f52f65643a2f7594/build-end.log



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