selftests/mm: include strings.h for ffsl

Got a compilation error on Android for ffsl after 91b80cc
("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: af605d2 ("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>

Author: Edward Liaw

tools/testing/selftests/mm/vm_util.h

@@ -3,7 +3,7 @@
 #include <stdbool.h>
 #include <sys/mman.h>
 #include <err.h>
-#include <string.h> /* ffsl() */
+#include <strings.h> /* ffsl() */
 #include <unistd.h> /* _SC_PAGESIZE */
 
 #define BIT_ULL(nr)                   (1ULL << (nr))