Proof of concept fix for Sonic not being MD2-able

As reported in http://mb.srb2.org/showthread.php?t=38632, MD2s and Linux don't mix too well.  InitMD2 is called before Sonic's skin can be registered in the Linux build, leading to Sonic's skin not being replacable.  This patch hacks around the problem by calling HWR_AddPlayerMD2 at the end of the function which registers Sonic's skin.  It is not an ideal fix, but it works for now at least.