BUG: Initializing class constants of type double is illegal C++.

- Fixes build failure in Visual Studio.

Libs/Widgets/ctkCrosshairLabel.cpp

@@ -55,9 +55,11 @@ public:
   ctkCrosshairLabel::CrosshairTypes CrosshairType;
   int       BullsEyeWidth;
 
-  static const double BULLS_EYE_BLANK_FRACTION = 0.1;
+  static const double BULLS_EYE_BLANK_FRACTION;
 };
 
+const double ctkCrosshairLabelPrivate::BULLS_EYE_BLANK_FRACTION = 0.1;
+
 // --------------------------------------------------------------------------
 // ctkCrosshairLabelPrivate methods