eliminate [-Wformat-zero-length] and [-Wunused-but-set-variable] warn…

…ing message

```
In file included from /__w/mros2-mbed/mros2-mbed/mros2/include/mros2.h:9,
                 from /__w/mros2-mbed/mros2-mbed/workspace/mturtle_teleop/app.cpp:17:
/__w/mros2-mbed/mros2-mbed/workspace/mturtle_teleop/app.cpp: In function 'int main()':
/__w/mros2-mbed/mros2-mbed/workspace/mturtle_teleop/app.cpp:116:18: warning: zero-length gnu_printf format string [-Wformat-zero-length]
  116 |       MROS2_INFO("");
      |                  ^~
/__w/mros2-mbed/mros2-mbed/mros2/include/mros2/logging.h:46:12: note: in definition of macro 'MROS2_INFO'
   46 |     printf(__VA_ARGS__); \
      |            ^~~~~~~~~~~
/__w/mros2-mbed/mros2-mbed/workspace/mturtle_teleop/app.cpp:88:10: warning: variable 'fds' set but not used [-Wunused-but-set-variable]
   88 |   pollfh fds[1];
      |          ^~~
/__w/mros2-mbed/mros2-mbed/workspace/mturtle_teleop/app.cpp:95:8: warning: unused variable 'c' [-Wunused-variable]
   95 |   char c;
      |        ^
```

workspace/mturtle_teleop/app.cpp

@@ -92,7 +92,6 @@ int main()
   auto publish_count = 10;
   double speed = 0.5;
   double turn = 1.0;
-  char c;
   while (1)
   {
     if (publish_count < 10)
@@ -111,9 +110,8 @@ int main()
       MROS2_INFO("q/z : increase/decrease max speeds by 10 percent");
       MROS2_INFO("w/x : increase/decrease only linear speed by 10 percent");
       MROS2_INFO("e/c : increase/decrease only angular speed by 10 percent");
-      MROS2_INFO("currently: speed %s / turn %s",
+      MROS2_INFO("currently: speed %s / turn %s\r\n",
                  double_to_string(speed).c_str(), double_to_string(turn).c_str());
-      MROS2_INFO("");
     }
 
     char c;