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/dma-helpers.c b/dma-helpers.c
index 499550f..c9620a5 100644
--- a/dma-helpers.c
+++ b/dma-helpers.c
@@ -11,6 +11,7 @@
 #include "trace.h"
 #include "qemu/range.h"
 #include "qemu/thread.h"
+#include "qemu/main-loop.h"
 
 /* #define DEBUG_IOMMU */