Fix a typo in a function prototype in PowerPCDisasm.h
Should be GetGPRName not GetGRPName.

Removed an unnecessary extern declaration of GetGRPName in RegisterWindow.cpp.

Also added GetFPRName to the PowerPCDisasm header for consistency.
Merge pull request #459 from lioncash/prototype-typo

Fix a typo in a function prototype in PowerPCDisasm.h