Fix a bug where you couldn't save individual packets from a file type that

Fix a bug where you couldn't save individual packets from a file type that
Wireshark doesn't support writing to.  The default in the drop-down menu
was libpcap, but since gtk_option_menu_set_history() doesn't send the
activate signal we set up earlier, you would have to manually click on the
drop-down and reselect libpcap for it to work.  Now it works as expected.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@22214 f5534014-38df-0310-8fa8-9805f1628bb7