Fix broken Linux server build.

llwindow refactoring in skinning-7 r117641 added dependency on
LLWindowCallbacks in llwindowheadless, which was not being built
for server only builds.

* Include llwindowcallbacks.cpp in all llwindow builds.
* Do not link with libatk and libsdl for server linux builds, since
  they are not needed and only packaged for the viewer.

Reviewed with James.