avoid the extern keyword in .cpp files

Source/Core/VideoCommon/BPStructs.cpp

@@ -11,6 +11,7 @@
 
 #include "VideoCommon/BPFunctions.h"
 #include "VideoCommon/BPStructs.h"
+#include "VideoCommon/Fifo.h"
 #include "VideoCommon/PerfQueryBase.h"
 #include "VideoCommon/PixelEngine.h"
 #include "VideoCommon/PixelShaderManager.h"
@@ -24,8 +25,6 @@
 
 using namespace BPFunctions;
 
-extern volatile bool g_bSkipCurrentFrame;
-
 static const float s_gammaLUT[] =
 {
 	1.0f,