[MIPS] Include cacheflush.h in uncache.c

This fixes this sparse warning:

arch/mips/lib/uncached.c:38:22: warning: symbol 'run_uncached' was not declared. Should it be static?

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>