MIPS: export has_transparent_hugepage() for modules

MIPS should export its local version of "has_transparent_hugepage"
so that loadable modules (dax) can use it.

Fixes this build error:
ERROR: modpost: "has_transparent_hugepage" [drivers/dax/dax.ko] undefined!

Fixes: fd8cfd30 ("arch: fix has_transparent_hugepage()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: linux-nvdimm@lists.01.org
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>