RFR: Fix Zero builds

[Aleksey Shipilev]

Zero builds currently fail with:

In file included from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp:28,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shenandoah/mode/shenandoahGenerationalMode.cpp:26:


   45 |       log_info(gc)("Heuristics ergonomically sets -XX:+" #name);            \
      |                ^~

   39 |   SHENANDOAH_ERGO_ENABLE_FLAG(ExplicitGCInvokesConcurrent);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/runtime/stackWatermarkSet.hpp:29,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/runtime/thread.hpp:42,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/runtime/safepoint.hpp:30,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shared/collectedHeap.hpp:35,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp:30,
                 from /home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.hpp:28,
   ... (rest of output omitted)

This is because we are missing the "log" includes (look at `shenandoahSATBMode.cpp` for comparison), which are not transitively available with Zero builds.

Additional testing: 
 - [x] Linux x86_64 Zero build (now passes)

-------------

Commit messages:
 - Fix Zero builds

Changes: https://git.openjdk.java.net/shenandoah/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=18&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/18/head:pull/18

PR: https://git.openjdk.java.net/shenandoah/pull/18