RFR(s): 8080585: concurrentGCThread.hpp should not include suspendibleThreadSet.hpp

[Per Liden]

Hi,

Here's another follow up patch to the GC directory restructure, which 
removes the unnecessary dependency between 
gc/shared/concurrentGCThread.hpp and gc/g1/suspendibleThreadSet.hpp

Webrev: http://cr.openjdk.java.net/~pliden/8080585/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8080585

cheers,
/Per