This line isn't necessary, as Processing includes it automatically when compiling.
But the error message is strange, nevertheless.

I removed the line and it works fine, and now I found another problem.
In the Processing update, it says using P3D in size function is the same as using OPENGL, so I replaced the P3D in the NyAR example and imported opengl library, however, doing this results in a grey screen. Any idea why this is happening? When I change back to P3D it works again...

On second thought, Processing analyses all the imports and check if it finds the referenced libraries. Apparently, it doesn't expect a reference to its own libraries, so it is confused...

I don't know NyAR, but perhaps it is really relying on P3D mode. Or perhaps you need to do some specific inits for OpenGL. I fear that future 2.0 will break a number of libraries, so I hope most of them will be updated soon...