Remove use of queue.h from gdbserver/event-loop.c

Remove use of queue.h from gdbserver/event-loop.c

This removes a use of queue.h from gdbserver/event-loop.c, replacing
it with std::queue.

I was not completely sure whether std::queue is even that useful.
Perhaps plain std::list could be used just as easily.

Tested by the buildbot.

gdb/gdbserver/ChangeLog
2018-06-09  Tom Tromey  <tom@tromey.com>

	* event-loop.c (gdb_event, gdb_event_p): Remove typedefs.  Don't
	declare queue.
	(event_queue): Use std::queue.
	(gdb_event_xfree): Remove.
	(initialize_event_loop, process_event, wait_for_event): Update.