Added plugin include and plugin install dirs to FindMagnum.cmake.
Added plugin include and plugin install dirs to FindMagnum.cmake.

ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum