libpcap: fix wireless handling

Based on ba23aa7b in libpcap upstream.

linux/wireless.h includes linux/if.h, which conflicts with net/if.h as
they both define if* structures. Fix build by simply using linux/if.h
instead of net/if.h.

The same fix should be done for the configure script, but cheat by
presetting it instead.

This fix in turn fixes kismet build with BR2_CONFIG_CACHE enabled, as
that uses the same ac_cv_linux_wireless_h cache value.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>