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_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index 71694a4..a45d344 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -36,13 +36,13 @@
 #include <osmocom/core/talloc.h>
 #include <osmocom/core/select.h>
 #include <osmocom/core/rate_ctr.h>
-#include <openbsc/gsm_04_08_gprs.h>
+#include <osmocom/gprs/gprs_bssgp.h>
 
+#include <openbsc/gsm_04_08_gprs.h>
 #include <openbsc/signal.h>
 #include <openbsc/debug.h>
 #include <openbsc/sgsn.h>
 #include <openbsc/gprs_llc.h>
-#include <openbsc/gprs_bssgp.h>
 #include <openbsc/gprs_sgsn.h>
 #include <openbsc/gprs_gmm.h>