[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 <ralf@linux-mips.org>