proper prototype for hugetlb_get_unmapped_area()

Add a proper prototype for hugetlb_get_unmapped_area() in
include/linux/hugetlb.h.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>