Don't assume that KeySym is always defined as long in wxGTK.

Apparently this is not the case when using Wayland and using "%ld" to print it
out results in an assert failure.

Closes #15351.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74573 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775