[zgc] Missing includes in gc/z/zTracer.cpp

Reproduce: configure and build with --with-jvm-features=-jfr

/data/jdk/src/hotspot/share/gc/z/zTracer.cpp: In member function 'virtual void ZYoungTracer::report_end(const Ticks&)':
/data/jdk/src/hotspot/share/gc/z/zTracer.cpp:105:40: error: incomplete type 'ZGeneration' used in nested name specifier
  105 | e.set_tenuringThreshold(ZGeneration::young()->tenuring_threshold());
      | ^~~~~
gmake[3]: *** [lib/CompileJvm.gmk:147: /data/jdk/build/linux-x86_64-server-fastdebug/hotspot/variant-server/libjvm/objs/zTracer.o] Error 1
gmake[3]: *** Waiting for unfinished jobs....
gmake[2]: *** [make/Main.gmk:252: hotspot-server-libs] Error 2