examples: use stderr for messages, stdout for data

Reported by: Karel Srot
Bug: https://bugzilla.redhat.com/867462

example/ssh2_echo.c

@@ -327,7 +327,7 @@ int main(int argc, char *argv[])
         }
 
         if (exitsignal)
-            printf("\nGot signal: %s\n", exitsignal);
+            fprintf(stderr, "\nGot signal: %s\n", exitsignal);
 
         libssh2_channel_free(channel);
         channel = NULL;