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/configure.ac b/openbsc/configure.ac
index 16eb732..3f9333a 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -114,6 +114,8 @@
 AC_OUTPUT(
     openbsc.pc
     include/openbsc/Makefile
+    include/osmocom/Makefile
+    include/osmocom/gprs/Makefile
     include/Makefile
     src/Makefile
     src/libtrau/Makefile