Support shared lib for libpcap.

libpcap is linked by netd (system/netd/server/Android.mk), tcpdump
(/external/tcpdump/Android.mk) and some verdors' components.

So using it as shared lib will be good for easy update and size
reduction.

Test: building succeeded, and the image was tested on angler.
Bug: 33056637
Change-Id: I1f346970ab69839155550bbc46fc89461679de1e