buildroot

wpa_supplicant: adjust make environment for pkg-config call

In case that wpa_supplicant is configured with DBus interface,
it calls pkg-config to find the dbus headers and libraries.
Adjust the make environment so that it will use the right
pkg-config search paths.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>