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/Makefile.am b/openbsc/include/osmocom/Makefile.am
new file mode 100644
index 0000000..596ce23
--- /dev/null
+++ b/openbsc/include/osmocom/Makefile.am
@@ -0,0 +1 @@
+SUBDIRS = gprs