aio / timers: Untangle include files

include/qemu/timer.h has no need to include main-loop.h and
doing so causes an issue for the next patch. Unfortunately
various files assume including timers.h will pull in main-loop.h.
Untangle this mess.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

thread-pool.c

@@ -23,6 +23,7 @@
 #include "block/block_int.h"
 #include "qemu/event_notifier.h"
 #include "block/thread-pool.h"
+#include "qemu/main-loop.h"
 
 static void do_spawn_thread(ThreadPool *pool);