SceneGraph: explicitly export also AbstractCamera symbols.
SceneGraph: explicitly export also AbstractCamera symbols.

Caused linker errors with Clang (GCC was fine).

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