mm/huge_memory.c: make __thp_get_unmapped_area static

__thp_get_unmapped_area is only used in mm/huge_memory.c.  Make it static.
Tested by building and booting the kernel.

Link: http://lkml.kernel.org/r/20190504102353.GA22525@bharath12345-Inspiron-5559


Signed-off-by: Bharath Vedartham <linux.bhar@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>