Disable -Wstrict-prototypes for logging_vty_add_cmds()

Since March 15th 2017, libosmocore API logging_vty_add_cmds() had its
parameter removed (c65c5b4ea0). However,
its declaration in C file didn't contain "(void)", which meant the
number of parameters is undefined and thus compiler doesn't complain.

However, in recent Change-Id I84fd99442d0cc400fa562fa33623c142649230e2
we changed the declaration to '(void)' to allow building with
-Wstrict-prototypes.  This caused downstream build-failures of osmo-mgw (1.4.0)
and osmo-e1-recorder (master).

So let's make one very small-scoped exception here.

Change-Id: Iadb54848b67db937b640dc2102dddb6e708a6a9f

include/osmocom/vty/logging.h

@@ -6,7 +6,12 @@
 #define FILTER_STR	"Filter log messages\n"
 
 struct log_info;
-void logging_vty_add_cmds(void);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+/* note this undefined argument declaration is intentional. There used
+ * to be an argument until 2017 which we no longer need .*/
+void logging_vty_add_cmds();
+#pragma GCC diagnostic pop
 void logging_vty_add_deprecated_subsys(void *ctx, const char *name);
 struct vty;
 struct log_target *osmo_log_vty2tgt(struct vty *vty);