Add g_game extern declaration to Core.hpp

Honestly not sure why it's not defined in there, but it's not up to me.

src/Core.hpp

@@ -87,7 +87,7 @@ public:
 
 // Top-level globally-accessible "god object" that runs the main loop and
 // provides global access to most other objects and many variables
-class Game {
+extern class Game {
 	char         unknown0x4[4];
 	unsigned     unknown0x8;
 	unsigned *   unknown0xC;
@@ -176,7 +176,7 @@ public:
 	virtual ~Game();
 
 	// TODO: methods
-};
+} * g_game;
 
 
 #endif

src/Jet2.cpp

@@ -2,6 +2,8 @@
 
 
 // Address: 0x0022FCE0
+// Core.cpp really seems like a more natural place to put this, but it doesn't
+// appear to be with Core.cpp's other data in .data
 Game * g_game;