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>
diff --git a/migration-exec.c b/migration-exec.c
index deab4e3..4790247 100644
--- a/migration-exec.c
+++ b/migration-exec.c
@@ -17,6 +17,7 @@
 
 #include "qemu-common.h"
 #include "qemu/sockets.h"
+#include "qemu/main-loop.h"
 #include "migration/migration.h"
 #include "migration/qemu-file.h"
 #include "block/block.h"