RFR: 8049411: Minimal VM build broken after gcId.cpp was added

[Stefan Karlsson]

Hi all,

Please, review this change to fix a build problem with the minimal VM.

http://cr.openjdk.java.net/~stefank/8049411/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8049411

When gcId.cpp was introduced, it wasn't added to the list of files to 
keep in the gc_implementation/shared directory.

thanks,
StefanK