iw: pkg-config cleanup

We no longer need to specify PKG_CONFIG_PATH since it's handled by the
host-pkgconf wrapper.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>