Fix uninitialized variable warning in wxMSW wxFileSystemWatcher code.

Fix uninitialized variable warning in wxMSW wxFileSystemWatcher code.

This warning was harmless as the variable was in fact always initialized if
the code where it was used was reached but g++ 4.6.1 is not smart enough to
see it, just as MSVC for which we already had a workaround. So initialize it
explicitly just to suppress the warning.

See #14459.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775