The slirp sub-module complains about not being able to find the glib
library on cross-compiles because it is using the default pkg-config
tool (which isn't installed in our cross-build docker images).
Preserve PKG_CONFIG in our host config and pass it down to slirp.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>