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>