wireshark: build with libcap on Linux (POSIX capabilities)

This makes running wireshark (or more specifically, dumpcap) as root a
bit more secure. From <wireshark-1.11.2>/doc/README.packaging:

  --with-libcap             If running as root, try to grab
                            CAP_NET_ADMIN and CAP_NET_RAW, then drop
                            privileges. Linux only.