Make wxSpinCtrlDouble::SetIncrement() update digits in wxGTK too

Follow the generic version and increase the number of digits if
necessary.

include/wx/gtk/spinctrl.h

@@ -243,7 +243,7 @@ public:
     void SetValue(const wxString& value) wxOVERRIDE        { wxSpinCtrlGTKBase::SetValue(value); } // visibility problem w/ gcc
     void SetValue(double value)                 { DoSetValue(value); }
     void SetRange(double minVal, double maxVal) { DoSetRange(minVal, maxVal); }
-    void SetIncrement(double inc)               { DoSetIncrement(inc); }
+    void SetIncrement(double inc);
     void SetDigits(unsigned digits);
 
     virtual int GetBase() const wxOVERRIDE { return 10; }