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/Makefile.am b/include/osmocom/msc/Makefile.am
index 9fa58c7..d98bc9c 100644
--- a/include/osmocom/msc/Makefile.am
+++ b/include/osmocom/msc/Makefile.am
@@ -1,8 +1,6 @@
 noinst_HEADERS = \
 	a_iface.h \
 	a_iface_bssap.h \
-	common.h \
-	common_cs.h \
 	db.h \
 	debug.h \
 	gsm_04_08.h \
@@ -18,10 +16,10 @@
 	iu_dummy.h \
 	mncc.h \
 	mncc_int.h \
+	msc_common.h \
 	msc_ifaces.h \
 	msc_mgcp.h \
 	a_reset.h \
-	osmo_msc.h \
 	ran_conn.h \
 	rrlp.h \
 	signal.h \