irqchip/gic-v3: Fix missing "__init" for gic_smp_init()

With an SMP configuration, gic_smp_init() calls set_smp_cross_call().
set_smp_cross_call() is marked with "__init".
So gic_smp_init() should also be marked with "__init".
gic_smp_init() is only called from gic_init_bases().
gic_init_bases() is also marked with "__init";
So marking gic_smp_init() with "__init" is fine.

Signed-off-by: Ingo Rohloff <ingo.rohloff@lauterbach.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200422112857.4300-1-ingo.rohloff@lauterbach.com