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/include/Makefile.am b/openbsc/include/Makefile.am
index 4596b6e..2cc8bee 100644
--- a/openbsc/include/Makefile.am
+++ b/openbsc/include/Makefile.am
@@ -1,3 +1,3 @@
-SUBDIRS = openbsc
+SUBDIRS = openbsc osmocom
 
 noinst_HEADERS = mISDNif.h compat_af_isdn.h