Common/DebugInterface: Make return value of GetColor() a u32

At its only usage point, its return value is stored into a u32, and the
default implementation returns 0xFFFFFFFF (-1), which would be an
unsigned integer. Given all of the bits are used to determine a color,
it makes slightly more sense to treat this as an unsigned value as
opposed to a signed one.

Source/Core/Common/DebugInterface.h

@@ -74,7 +74,7 @@ public:
   virtual void SetPC(u32 /*address*/) {}
   virtual void Step() {}
   virtual void RunToBreakpoint() {}
-  virtual int GetColor(u32 /*address*/) { return 0xFFFFFFFF; }
+  virtual u32 GetColor(u32 /*address*/) { return 0xFFFFFFFF; }
   virtual std::string GetDescription(u32 /*address*/) = 0;
   virtual void Clear() = 0;
 };