wireshark: build with libcap (POSIX capabilities)

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

  The "--with-libcap" option is only useful when dumpcap is installed
  setuid. If it is enabled dumpcap will try to drop any setuid privileges
  it may have while retaining the CAP_NET_ADMIN and CAP_NET_RAW
  capabilities. It is enabled by default, if the Linux capabilities
  library (on which it depends) is found.