Because the Inverted-F antenna performances (and of almost any printed antenna) depends by the substrate Er, substrates thickness, ground placement, ground dimensions, etc., most probably you will not find in the library of a PCB design software, a generic printed antenna that will work in all cases.
Have to design first the antenna using an EM simulator (2D at least) using your PCB information (Er, thickness, ground, etc) and later after you get the dimensions information you can design your own antenna in Eagle, saving it as an independent package.