haiku libGL: Move from gallium target to src/hgl

haiku libGL: Move from gallium target to src/hgl

* The Haiku renderers need to link to libGL to function properly
  in all usage contexts. As mesa drivers build before gallium
  targets, we couldn't properly link the mesa swrast driver to
  the gallium libGL target for Haiku.
* This is likely better as it mimics how glx is laid out ensuring
  the Haiku libGL is better understood.
* All renderers properly link in libGL now.

Acked-by: Brian Paul <brianp@vmware.com>