Fix build error on windows

I'm not totally sure what's up, but the double declaration doesn;t work:

canning dependencies of target kritaui
[ 55%] Building CXX object
libs/ui/CMakeFiles/kritaui.dir/widgets/KisScreenColorPicker.cpp.obj
C:\dev\krita\libs\ui\widgets\KisScreenColorPicker.cpp:275:83: warning:
'KisDlgInternalColorSelector::s_screenColorPickerFactory' redeclared
without dllimport attribute: previous dllimport ignored [-Wattributes]
std::function<KisScreenColorPickerBase *(QWidget *)>
KisDlgInternalColorSelector::s_screenColorPickerFactory =
KisScreenColorPicker::createScreenColorPicker;

   ^~~~~~~~~~~~~~~~~~~~~~~~~~

C:\dev\krita\libs\ui\widgets\KisScreenColorPicker.cpp: In function 'void
__tcf_3()':
C:\dev\krita\libs\ui\widgets\KisScreenColorPicker.cpp:275:54: error:
definition of static data member
'KisDlgInternalColorSelector::s_screenColorPickerFactory' of dllimport'd
class
std::function<KisScreenColorPickerBase *(QWidget *)>
KisDlgInternalColorSelector::s_screenColorPickerFactory =
KisScreenColorPicker::createScreenColorPicker;

^~~~~~~~~~~~~~~~~~~~~~~~~~~

libs\ui\CMakeFiles\kritaui.dir\build.make:6050: recipe for target
'libs/ui/CMakeFiles/kritaui.dir/widgets/KisScreenColorPicker.cpp.obj'
failed
mingw32-make[2]: *
[libs/ui/CMakeFiles/kritaui.dir/widgets/KisScreenColorPicker.cpp.obj]
Error 1
CMakeFiles\Makefile2:18278: recipe for target
'libs/ui/CMakeFiles/kritaui.dir/all' failed
mingw32-make[1]: * [libs/ui/CMakeFiles/kritaui.dir/all] Error 2
Makefile:139: recipe for target 'all' failed
mingw32-make: *** [all] Error 2