package/ulogd: add optional support for libpcap

When libpcap was compiled before, ulogd will use it as optional
dependency:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/ulogd/ulogd_output_PCAP.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libpcap.so.1]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]

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