split libgb into a separate library for outside use

This also removes the dependency to osmo_sock() inside libcommon and
replaces it with osmo_sock_* from libosmocore
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index f7eb221..640388d 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -38,6 +38,9 @@
 #include <osmocom/core/rate_ctr.h>
 #include <osmocom/core/logging.h>
 
+#include <osmocom/gprs/gprs_ns.h>
+#include <osmocom/gprs/gprs_bssgp.h>
+
 #include <osmocom/vty/telnet_interface.h>
 #include <osmocom/vty/logging.h>
 
@@ -45,8 +48,6 @@
 #include <openbsc/debug.h>
 #include <openbsc/vty.h>
 #include <openbsc/sgsn.h>
-#include <openbsc/gprs_ns.h>
-#include <openbsc/gprs_bssgp.h>
 #include <openbsc/gprs_llc.h>
 
 #include <gtp.h>