MIPS: Fix warning when including smp-ops.h with CONFIG_SMP=n

The gic_send_ipi_mask function declared in smp-ops.h takes a struct
cpumask argument, but linux/cpumask.h is only included within an #ifdef
CONFIG_SMP. Move the gic_ function declarations within that #ifdef too
to fix warnings during build such as:

In file included from arch/mips/fw/arc/init.c:15:0:
/mnt/buildbot/kernel/mips/slave/mips-linux__allno_/build/arch/mips/include/asm/smp-ops.h:62:44:
warning: 'struct cpumask' declared inside parameter list [enabled by
default]
 extern void gic_send_ipi_mask(const struct cpumask *mask, unsigned int
action);

Reported-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6655/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>