tests: fix compilation error for os400

OS400 uses BSD 4.3 setsockopt() prototype by default: this does not
define parameter as const, resulting in an error if actual parameter is
const. Remove the const keyword from the actual parameter cast: this
works in all conditions, even if the formal parameter uses it.

Closes #11547

tests/libtest/lib1522.c

@@ -40,7 +40,7 @@ static int sockopt_callback(void *clientp, curl_socket_t curlfd,
   (void) clientp;
   (void) purpose;
   setsockopt(curlfd, SOL_SOCKET, SO_SNDBUF,
-             (const char *)&sndbufsize, sizeof(sndbufsize));
+             (char *)&sndbufsize, sizeof(sndbufsize));
 #else
   (void)clientp;
   (void)curlfd;