greybus: audio_topology: Fix compile warning

Fix following compile warning by staticizing gb_generate_enum_strings().

greybus/audio_topology.c:134:12: warning: symbol 'gb_generate_enum_strings' was not declared. Should it be static?
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/audio_topology.c

@@ -131,7 +131,7 @@ static const char *gbaudio_map_widgetid(struct gbaudio_module_info *module,
 	return NULL;
 }
 
-const char **gb_generate_enum_strings(struct gbaudio_module_info *gb,
+static const char **gb_generate_enum_strings(struct gbaudio_module_info *gb,
 					     struct gb_audio_enumerated *gbenum)
 {
 	const char **strings;