gui2: Ignore SDL_SYSWMEVENT events on X11

These events are emitted for clipboard functionality after setting
Wesnoth as the clipboard owner, but they are already handled in
src/event.cpp. GUI2 handling them too results in unnecessary extra calls
to our handle_system_event() function, which may lead to the clipboard
being cleared, at least on KDE.

This commit fixes this issue by ignoring SDL_SYSWMEVENT on X11 too in
X11 land, instead of ignoring it only on Win32. No idea why it was
specifically ignored on Win32 only and not on every other platform that
isn't X11 to begin with, though. Hopefully mordante will review this
commit later.