build: Make -Werror=int-conversion fatal
build: Make -Werror=int-conversion fatal

This should hopefully avoid me accidentally passing NULL for an `int`
file descriptor in the future.

Closes: #431
Approved by: giuseppe

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