Include migration/vmstate.h less

In my "build everything" tree, changing migration/vmstate.h triggers a
recompile of some 2700 out of 6600 objects (not counting tests and
objects that don't depend on qemu/osdep.h).

hw/hw.h supposedly includes it for convenience.  Several other headers
include it just to get VMStateDescription.  The previous commit made
that unnecessary.

Include migration/vmstate.h only where it's still needed.  Touching it
now recompiles only some 1600 objects.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190812052359.30071-16-armbru@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
diff --git a/hw/timer/imx_gpt.c b/hw/timer/imx_gpt.c
index b4cfc52..49a441f 100644
--- a/hw/timer/imx_gpt.c
+++ b/hw/timer/imx_gpt.c
@@ -15,6 +15,7 @@
 #include "qemu/osdep.h"
 #include "hw/irq.h"
 #include "hw/timer/imx_gpt.h"
+#include "migration/vmstate.h"
 #include "qemu/main-loop.h"
 #include "qemu/module.h"
 #include "qemu/log.h"