RFR(trivial): JDK-8221763: Build fails when using --with-jvm-features=-g1gc, -jfr, -shenandoahgc

[Ao Qi]

Hi,

Build is broken when configure with
--with-jvm-features=-g1gc,-jfr,-shenandoahgc, since
gc/shared/softRefPolicy.hpp is missed.

Could you please review this trivial patch?

Bug:
https://bugs.openjdk.java.net/browse/JDK-8221763

Webrev:
http://cr.openjdk.java.net/~aoqi/8221763/webrev.00

Tested:
linux-x86_64-{server, minimal, zero}-{fastdebug, release} build,
linux-x86_64-{server, minimal, zero}-{fastdebug, release} build
(configure with -g1gc,-jfr,-shenandoahgc), linux-x86_64-server-release
hotspot:tier1_gc

Thanks,
Ao Qi