Make libcommon, libcommon-cs, libfilter, utils depend on mgcp/sigtran

The stow-enabled jenkins builds are currently failing like below:

make[3]: Entering directory '/build/src/libcommon'
  CC       bsc_version.o
  CC       common_vty.o
  CC       debug.o
  CC       gsm_data.o
In file included from debug.c:34:0:
../../include/osmocom/bsc/gsm_data.h:15:38: fatal error: osmocom/sigtran/sccp_sap.h: No such file or directory
 #include <osmocom/sigtran/sccp_sap.h>
                                      ^
compilation terminated.
In file included from common_vty.c:27:0:
../../include/osmocom/bsc/gsm_data.h:15:38: fatal error: osmocom/sigtran/sccp_sap.h: No such file or directory
 #include <osmocom/sigtran/sccp_sap.h>
                                      ^
compilation terminated.
In file included from gsm_data.c:37:0:
../../include/osmocom/bsc/gsm_data.h:15:38: fatal error: osmocom/sigtran/sccp_sap.h: No such file or directory
 #include <osmocom/sigtran/sccp_sap.h>
                                      ^
compilation terminated.

Let's make sure the common gsm_data.h have all required CFLAGS to compile.

Change-Id: I30b75db6ffba227b05b5413b84b15f69e0c213f2
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 0ff5811..ed2a95c 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -12,12 +12,14 @@
 #include <osmocom/core/stats.h>
 
 #include <osmocom/crypt/auth.h>
-#include <osmocom/sigtran/sccp_sap.h>
 
 #include <osmocom/bsc/common.h>
 #include <osmocom/bsc/rest_octets.h>
 #include <osmocom/bsc/common_cs.h>
 
+struct mgcp_client_conf;
+struct mgcp_client;
+struct mgcp_ctx;
 
 /** annotations for msgb ownership */
 #define __uses