powerpc/mce: Make symbol 'mce_ue_event_work' static

The sparse tool complains as follows:

arch/powerpc/kernel/mce.c:43:1: warning:
 symbol 'mce_ue_event_work' was not declared. Should it be static?

This symbol is not used outside of mce.c, so this commit marks it
static.

Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210408035802.31853-1-lihuafei1@huawei.com