powerpc/64s/radix: Fix build failure with RADIX_MMU=n

powerpc/64s/radix: Fix build failure with RADIX_MMU=n

After merging the powerpc tree, today's linux-next build (powerpc64
allnoconfig) failed like this:

 arch/powerpc/mm/book3s64/pgtable.c:216:3:
 error: implicit declaration of function 'radix__flush_all_lpid_guest'

radix__flush_all_lpid_guest() is only declared for
CONFIG_PPC_RADIX_MMU which is not set for this build.

Fix it by adding an empty version for the RADIX_MMU=n case, which
should never be called.

Fixes: 99161de3a283 ("powerpc/64s/radix: tidy up TLB flushing code")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[mpe: Munge change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190930101342.36c1afa0@canb.auug.org.au