avoid the extern keyword in .cpp files

Source/Core/VideoBackends/Software/SWCommandProcessor.h

@@ -11,9 +11,6 @@
 class PointerWrap;
 namespace MMIO { class Mapping; }
 
-extern volatile bool g_bSkipCurrentFrame;
-extern u8* g_pVideoData;
-
 namespace SWCommandProcessor
 {
 	using UCPStatusReg = CommandProcessor::UCPStatusReg;