Avoid the use of undefined typedefs

When using the macro **PCL_ADD_RGB* outside of **pcl::**, I get a compiler error because that macro uses typedefs (**Vector3c, Vector3cMap, Vector3cMapConst, Vector4c, Vector4cMap and Vector4cMapConst**) which are so far undefined. All this occurs inside **point_types.hpp**. A possible solution is to just move the typedefs above the definition of **PCL_ADD_RGB**, but this still doesn't allow users to use the macro outside of the **pcl::**.

For instance, creating a new external project (http://pointclouds.org/documentation/tutorials/using_pcl_pcl_config.php) with an executable with the following code would produce an error.

```c++
#include <pcl/point_types.h>

struct MyRGBType
{
  PCL_ADD_RGB;
};

int main (int argc, char** argv)
{
  return (0);
}
```

This PR only changes the **PCL_ADD_EIGEN_MAPS_RGB** to not use the typedefs. Then the above works.