VideoCommon/OpcodeDecoding: Move g_bRecordFifoData into namespace

Keeps the global localized with the code that it's primarily related to.

Now it's obvious from a glance what the global variable is affecting.

Source/Core/VideoCommon/OpcodeDecoding.h

@@ -10,6 +10,9 @@ class DataReader;
 
 namespace OpcodeDecoder
 {
+// Global flag to signal if FifoRecorder is active.
+extern bool g_record_fifo_data;
+
 enum
 {
   GX_NOP = 0x00,