buildroot

reaver: fix static link build issue

When libpcap itself is linked against other libraries, reaver fails to
build as it doesn't link with libpcap dependencies. This patch fixes
that by using the pcap-config program.

Fixes:

  http://autobuild.buildroot.net/results/899fd633288d5cd5aa221413cded857e4f743194/

Signed-off-by: Guillaume W. Bres <guillaume.bressaix@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>