Fix wxGTK1 event loop definition mismatch between base/GUI.

Fix wxGTK1 event loop definition mismatch between base/GUI.

We must define wxEventLoopBase in the same way in base and GUI code,
otherwise, even though we fool the compiler into accepting our code, it
crashes or behaves otherwise weirdly during run-time because of vtbl mismatch.

This fixes wxGTK1 which was crashing on startup since the FSWATCHER branch
merge in r62474 and associated changes to support the event loop sources.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64146 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775