gcc 4 doesn't like my macro for getting the interface description: it says there...

gcc 4 doesn't like my macro for getting the interface description: it says there's an invalid lvalue in it.  I don't see what's wrong with it, but replace the macro with a function.  Also: don't include the .xpm files in gtk/main.c if we're building without PCAP (as they're not used in that configuration).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@22588 f5534014-38df-0310-8fa8-9805f1628bb7