rofiles-fuse: Fix truncate call to not use O_CREAT
rofiles-fuse: Fix truncate call to not use O_CREAT

I was getting a compilation error with the GCC hardening flags which
look for a missing mode with `O_CREAT`.  The right fix here is to drop
`O_CREAT`, as truncate() should throw `ENOENT` if the file doesn't
exist.

GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree