wireshark: fix static build

Don't give a path in --with-pcap option otherwise PCAP_CONFIG won't be
used (see AC_WIRESHARK_PCAP_CHECK in acinclude.m4) to retrieve pcap
static dependencies such as -lnl-3 and as a result pcap detection will
fail

Fixes:
 - http://autobuild.buildroot.org/results/084354a33825cf4e723e0a9f1e9656ad2a212a15



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>