alpha: Fix fall-out from disintegrating asm/system.h

Commit ec221208 ("Disintegrate asm/system.h for Alpha") removed
asm/system.h however arch/alpha/oprofile/common.c requires definitions
that were shifted from asm/system.h to asm/special_insns.h.  Include
that.

Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>