migration: Export exec.c functions in its own file

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/migration/exec.c b/migration/exec.c
index 57a9335..9077024 100644
--- a/migration/exec.c
+++ b/migration/exec.c
@@ -21,6 +21,7 @@
 #include "qapi/error.h"
 #include "qemu-common.h"
 #include "channel.h"
+#include "exec.h"
 #include "migration/migration.h"
 #include "io/channel-command.h"
 #include "trace.h"