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/osmocom/gprs/Makefile.am b/openbsc/include/osmocom/gprs/Makefile.am
new file mode 100644
index 0000000..fe5714e
--- /dev/null
+++ b/openbsc/include/osmocom/gprs/Makefile.am
@@ -0,0 +1,3 @@
+libgb_HEADERS = gprs_bssgp.h gprs_ns.h gprs_ns_frgre.h
+
+libgbdir = $(includedir)/osmocom/gprs