combine several small .h in msc_common.h

For hysterical raisins, there are some header files that contain few
declarations, and where the name doesn't reflect the content. Combine them to
new msc_common.h:

- common.h
- common_cs.h
- osmo_msc.h

Change-Id: I9e3a587342f8d398fb27354a2f2475f8797cdb28
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 35ec752..3ffa100 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -13,10 +13,10 @@
 #include <osmocom/gsm/gsm48.h>
 #include <osmocom/crypt/auth.h>
 
-#include <osmocom/msc/common.h>
-#include <osmocom/msc/common_cs.h>
 #include <osmocom/mgcp_client/mgcp_client.h>
 
+#include <osmocom/msc/msc_common.h>
+
 #include "gsm_data_shared.h"
 
 /* TS 48.008 DLCI containing DCCH/ACCH + SAPI */