avoid the extern keyword in .cpp files

Source/Core/VideoBackends/Software/OpcodeDecoder.cpp

@@ -14,6 +14,7 @@
 #include "VideoBackends/Software/SWVideoConfig.h"
 #include "VideoBackends/Software/XFMemLoader.h"
 #include "VideoCommon/DataReader.h"
+#include "VideoCommon/Fifo.h"
 
 typedef void (*DecodingFunction)(u32);