Enable stdout autoflush in authsock.pl

With enabled buffering the output gets lost when the process
receives a TERM signal. Disable the buffering.

(cherry picked from commit a0311dfb6e2a51f89dfa8b200b96a0f4675fb654)

bin/tests/system/tsiggss/authsock.pl

@@ -31,6 +31,10 @@ if (!defined($path)) {
 	exit(1);
 }
 
+# Enable output autoflush so that it's not lost when the parent sends TERM.
+select STDOUT;
+$| = 1;
+
 unlink($path);
 my $server = IO::Socket::UNIX->new(Local => $path, Type => SOCK_STREAM, Listen => 8) or
     die "unable to create socket $path";