qemu: needs pkg-config

Fixes

  http://autobuild.buildroot.org/results/ccb7d6da503a81015275ff1e4ba9a564203932ec/build-end.log



Even though the build failure happens on host-qemu, adding
host-pkgconf to QEMU_DEPENDENCIES seems like the right thing to do
since that this dependency will also be needed for the target qemu
once it gets included.

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