clocksource/drivers/loongson1: Set variable ls1x_timer_lock storage-class-specifier to static

smatch reports
drivers/clocksource/timer-loongson1-pwm.c:31:1: warning: symbol
  'ls1x_timer_lock' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230702133113.3438049-1-trix@redhat.com