selftests: safesetid: Fix spelling mistake "cant" -> "can't"

There is a spelling mistake in an error message. Fix it.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/safesetid/safesetid-test.c

@@ -152,7 +152,7 @@ static void write_policies(void)
 
 	fd = open(add_whitelist_policy_file, O_WRONLY);
 	if (fd < 0)
-		die("cant open add_whitelist_policy file\n");
+		die("can't open add_whitelist_policy file\n");
 	written = write(fd, policy_str, strlen(policy_str));
 	if (written != strlen(policy_str)) {
 		if (written >= 0) {