Do not generally add a plugin's directory to its include paths.

It's not necessary, and it lets people get away with using
the wrong kind of include statement.

Change-Id: I736a0aae93c930da37dbac548a3bb025838e7588
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>