use new external libosmogb (part of libosmocore.git)

This removes the libgb (GPRS NS/BSSGP) code from the openbsc.git
repository and uses the new version from libosmocore.git instead.
diff --git a/openbsc/include/Makefile.am b/openbsc/include/Makefile.am
index 2cc8bee..4596b6e 100644
--- a/openbsc/include/Makefile.am
+++ b/openbsc/include/Makefile.am
@@ -1,3 +1,3 @@
-SUBDIRS = openbsc osmocom
+SUBDIRS = openbsc
 
 noinst_HEADERS = mISDNif.h compat_af_isdn.h