systemc: Include eventq_impl.hh in scheduler.hh.

systemc: Include eventq_impl.hh in scheduler.hh.

This ensures that we also get the inline definitions of some of the
methods defined in the EventQueue class. In certain circumstances gem5
won't link properly otherwise.

Change-Id: Ie0dfef207a165095bdfe1199cd1f690cebc4cbbf
Issue-on: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-597
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29397
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>