ppc: make PPC_BOOK3S_64 select IRQ_WORK

Fix breakage which will be exposed by the patch "kconfig: make allnoconfig
disable options behind EMBEDDED and EXPERT".

arch/powerpc/kernel/mce.c, compiled in for PPC_BOOK3S_64, calls
functions only built when IRQ_WORK, so select it.  Fixes the following
build error:

  arch/powerpc/kernel/built-in.o: In function `.machine_check_queue_event':
  (.text+0x11260): undefined reference to `.irq_work_queue'

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>