greybus: control: Fix sparse warnings

gb_control_get_version() is not used outside of the file and must be
marked as static. Following sparse warnings are reported today:

greybus/control.c:20:5: warning: symbol 'gb_control_get_version' was not declared. Should it be static?

Fix it by marking gb_control_get_version() 'static'.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>