libgb: remove dependencies to openbsc/vty.h and openbsc/gsm_data.h

Rather than using openbsc internal data/functions, we now use only
internal and libosmocore-provided ones.
diff --git a/openbsc/src/libgb/common_vty.h b/openbsc/src/libgb/common_vty.h
new file mode 100644
index 0000000..a278266
--- /dev/null
+++ b/openbsc/src/libgb/common_vty.h
@@ -0,0 +1,5 @@
+#include <osmocom/vty/command.h>
+
+extern struct cmd_element libgb_exit_cmd;
+extern struct cmd_element libgb_end_cmd;
+