docs: Define G_OS_UNIX and G_OS_WIN32 unconditionally
docs: Define G_OS_UNIX and G_OS_WIN32 unconditionally

* docs/reference/Doxyfile.in (PREDEFINED): Always define both
G_OS_UNIX and G_OS_WIN32, so that the generated documentation
covers all platforms.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm