userfaultfd: selftest: fix compiler warning

Fixes following compiler warning

userfaultfd.c: In function ‘usage’:
userfaultfd.c:126:2: warning: format not a string literal and no format
	arguments [-Wformat-security]
  fprintf(stderr, examples);
Signed-off-by: Alakesh Haloi <alakesh.haloi@gmail.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -123,7 +123,7 @@ static void usage(void)
 	fprintf(stderr, "Supported <test type>: anon, hugetlb, "
 		"hugetlb_shared, shmem\n\n");
 	fprintf(stderr, "Examples:\n\n");
-	fprintf(stderr, examples);
+	fprintf(stderr, "%s", examples);
 	exit(1);
 }