RFR (XXS): 8016170: GC id variable in gcTrace.cpp should use typedef GCId

[Erik Helin]

Hi all,

this very small change ensures that variable GCTracer_next_gc_id in
gcTrace.cpp always has the same type as the typedef GCId in gcTrace.hpp.

Webrev:
http://cr.openjdk.java.net/~ehelin/8016170/webrev.00/

Testing:
JPRT

Thanks,
Erik