[MIPS] Fix warning about unused definition in c-sb1.c

arch/mips/mm/c-sb1.c: In function `sb1_cache_init':
arch/mips/mm/c-sb1.c:447: warning: unused variable `handle_vec2_sb1'

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>