libpcap: omit -rpath from pcap-config

For target processes the libraries are always in their usual location under
/lib and /usr/lib. It doesn't make sense to add RPATH tag pointing to host
build directory to programs using libpcap. This commit changes V_RPATH_OPT in
pcap-config to remove -rpath from its output.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>