Add an #ifdef G_OS_UNIX around the GLIB_AVAILABLE_IN_ALL annotation on
the _get_type() functions for GLocal{File,Directory}Monitor.

These symbols are in private header files and are only exported so that
the in-tree file monitoring modules can subclass.  This is only needed
on UNIX and was therefore never part of the public ABI on Windows.

Caught by Dieter Verfaillie.