vty/command: add 'hidden only' VTY reference generation mode

Change-Id: I511ce26350cd04bb0f66d130b5286cab84f16df2
Related: SYS#4910
diff --git a/include/osmocom/vty/command.h b/include/osmocom/vty/command.h
index b608848..add57e2 100644
--- a/include/osmocom/vty/command.h
+++ b/include/osmocom/vty/command.h
@@ -464,6 +464,8 @@
 	VTY_REF_GEN_MODE_DEFAULT = 0,
 	/*! Expert mode: all commands including hidden, excluding deprecated. */
 	VTY_REF_GEN_MODE_EXPERT,
+	/*! "Inverse" mode: only hidden commands. */
+	VTY_REF_GEN_MODE_HIDDEN,
 };
 
 extern const struct value_string vty_ref_gen_mode_names[];