[MIPS] SB1: Fix modpost warning.

MODPOST vmlinux
WARNING: arch/mips/mm/built-in.o(.text+0x1978): Section mismatch: reference to .init.text:except_vec2_sb1 (between 'sb1_cache_init' and 'sb1_flush_cache_sigtramp')
WARNING: arch/mips/mm/built-in.o(.text+0x1988): Section mismatch: reference to .init.text:except_vec2_sb1 (between 'sb1_cache_init' and 'sb1_flush_cache_sigtramp')

Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/mm/c-sb1.c

···
  * memory management function pointers, as well as initialize
  * the caches and tlbs
  */
-void sb1_cache_init(void)
+void __init sb1_cache_init(void)
 {
 	extern char except_vec2_sb1;