ARM: gic: enclose gic_sgi_lock declaration with CONFIG_SMP ifdef

Add dependency on CONFIG_SMP to the declaration of gic_sgi_lock
since the users of the lock have the same.

Change-Id: I0175d415d8e3e711918677f9433113aa2db51bda
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>