powerpc: Add missing prototype for slb_miss_bad_addr()

In commit f0f558b1 ("powerpc/mm: Preserve CFAR value on SLB miss caused
by access to bogus address"), the function slb_miss_bad_addr() was added
without a prototype. This commit adds it.

Fix a warning (treated as error in W=1):
  arch/powerpc/kernel/traps.c:1498:6: error: no previous prototype for ‘slb_miss_bad_addr’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>