package/wireshark: Fix libpcap detection

To reproduce the build error I had to install libpcap0.8-dev on my host
system, then wireshark configure picks up

checking for pcap-config... /usr/bin/pcap-config

and fails.

Fixes
http://autobuild.buildroot.net/results/7e3/7e363d18866057df0db3d0d95fa8d9116728f6ce//


and many others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>