examples: fix build on macOS with gtk+-quartz-3.0
gdk_quartz_window_get_nsview is not declared in the header file now: error: implicit declaration of function 'gdk_quartz_window_get_nsview' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
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gdk_quartz_window_get_nsview is not declared in the header file now: error: implicit declaration of function 'gdk_quartz_window_get_nsview' is invalid in C99 [-Werror,-Wimplicit-function-declaration]