Zero builds fail after JDK-8255984

This went through testing cracks, as JDK-8256497 was integrated at the same time. Zero x86_64 fails with:

In file included from /home/runner/work/jdk/jdk/jdk/src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.hpp:30,
                 from /home/runner/work/jdk/jdk/jdk/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp:28:
/home/runner/work/jdk/jdk/jdk/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp:348:17: error: inline function 'size_t ShenandoahHeapRegion::get_live_data_bytes() const' used but never defined [-Werror]
  348 | inline size_t get_live_data_bytes() const;
      | ^~~~~~~~~~~~~~~~~~~
/home/runner/work/jdk/jdk/jdk/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp:351:17: error: inline function 'size_t ShenandoahHeapRegion::garbage() const' used but never defined [-Werror]
  351 | inline size_t garbage() const;
      | ^~~~~~~

The fix is trivial: reinstating the #include.