staging: lustre: lustre: lov: lov_dev: fix sparse warning of static declaration

This patch adds a static keyword to cl_lov_device_mutex_class variable
to suppress the warning of static declaration

Signed-off-by: Mohammad Jamal <md.jamalmohiuddin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>