Add host-pkg-config to linux configuration

In case of passing pkg_config also set host_pkg_config
otherwise pkg-config for sysroot will inject sysroot root libdir
linker flag during host build which leads to weird errors.

Change-Id: I57f277e500ba175687f65b9947b9f9c8edd969fc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>