parisc: Define HAVE_ARCH_HUGETLB_UNMAPPED_AREA

Define the HAVE_ARCH_HUGETLB_UNMAPPED_AREA macro like other platforms do in
their page.h files to avoid this compile warning:
arch/parisc/mm/hugetlbpage.c:25:1: warning: no previous prototype for 'hugetlb_get_unmapped_area' [-Wmissing-prototypes]

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org  # 6.0+
Reported-by: John David Anglin <dave.anglin@bell.net>
Tested-by: John David Anglin <dave.anglin@bell.net>