RFR: 8142390: Move ScanRSClosure to header file

[Erik Helin]

Hi all,

this small patches move the declaration of ScanRSClosure from
g1RemSet.cpp to g1RemSet.hpp.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142390

Webrev:
http://cr.openjdk.java.net/~ehelin/8142390/webrev.00/

Testing:
JPRT

Thanks,
Erik