migration: Split migration/channel.c for channel operations

Create an include for its exported functions.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

---
Add proper header
diff --git a/migration/tls.c b/migration/tls.c
index a33ecb7..34ad121 100644
--- a/migration/tls.c
+++ b/migration/tls.c
@@ -19,6 +19,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "channel.h"
 #include "migration/migration.h"
 #include "io/channel-tls.h"
 #include "crypto/tlscreds.h"