mm, x86: fix HIGHMEM64 && PARAVIRT build config for native_pud_clear()

Looks like I also missed the build config that includes
CONFIG_HIGHMEM64G && CONFIG_PARAVIRT to export the native_pud_clear()
dummy function.

Fixes: a00cc7d9

 ("mm, x86: add support for PUD-sized transparent hugepages")
Link: http://lkml.kernel.org/r/148823188084.56076.17451228917824355200.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reported-by: Laura Abbott <labbott@redhat.com>
Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>