selftests/mm: include strings.h for ffsl

selftests/mm: include strings.h for ffsl

commit 176517c9310281d00dd3210ab4cc4d3cdc26b17e upstream.

Got a compilation error on Android for ffsl after 91b80cc5b39f
("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: af605d26a8f2 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>