Fix missing namespace for string

[Tobias-Fischer]

No description provided.

[Tobias-Fischer]

Fixes compile issue

$SRC_DIR/gazebo/msgs/generator/GazeboGenerator.hh:40:23: error: 'string' does not name a type; did you mean 'stdin'?
   40 |                 const string& parameter,
      |                       ^~~~~~
      |                       stdin
$SRC_DIR/gazebo/msgs/generator/GazeboGenerator.hh:42:17: error: 'string' has not been declared
   42 |                 string* error) const;
      |                 ^~~~~~

[traversaro]

For reference, this is the Classic Gazebo version of the fix gazebosim/gz-msgs#242, it is useful for Protobuf 3.20 compatibility.