Fix logic error in OpenCVFindLibsGUI.cmake
Fix logic error in OpenCVFindLibsGUI.cmake

When with_gtk is selected but GTK3 is not present the current logic
fails to check for GTK2. This edit corrects this.

opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv