Add missing dependency on pluginlib

Nodelet does not pass along pluginlib implicitly, because pluginlib is not used in any exposed headers.  This is correct behavior on nodelet's behalf.

This library directly imports pluginlib:

https://github.com/ros-perception/laser_proc/blob/groovy-devel/src/LaserProcNodelet.cpp#L59

And therefore this package should depend on pluginlib explicitly.