selftests/mm: include strings.h for ffsl

Got a compilation error on Android for ffsl after 91b80cc5
("selftests: mm: fix map_hugetlb failure on 64K page size systems")
included vm_util.h.

Link: https://lkml.kernel.org/r/20240329185814.16304-1-edliaw@google.com


Fixes: af605d26 ("selftests/mm: merge util.h into vm_util.h")
Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>