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/net/socket.c b/net/socket.c
index 87af1d3..e61309d 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -31,6 +31,7 @@
 #include "qemu/option.h"
 #include "qemu/sockets.h"
 #include "qemu/iov.h"
+#include "qemu/main-loop.h"
 
 typedef struct NetSocketState {
     NetClientState nc;