powerpc/fsl_rio: Fix compile error when CONFIG_FSL_RIO not set

arch/powerpc/kernel/built-in.o: In function `machine_check_e500mc':
arch/powerpc/kernel/traps.c:429: undefined reference to `fsl_rio_mcheck_exception'
arch/powerpc/kernel/built-in.o: In function `machine_check_e500':
arch/powerpc/kernel/traps.c:519: undefined reference to `fsl_rio_mcheck_exception'
make: *** [.tmp_vmlinux1] Error 1

Reported-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>