GLocalFile: canonicalize the initial directory separator

GLocalFile was (in certain situations) translating a path like
"/foo/bar/baz" to "/foo\bar\baz" on win32. Fix it to make sure the
initial directory separator gets canonicalized too.

Fixes gio/tests/g-icon on win32.

https://bugzilla.gnome.org/show_bug.cgi?id=688109