src/udev/Makefile.am: do not fail if sym link already exists

This fixes two issues: 1) The -r option does not exist for busybox
and we do not need relative sym linking anyhow, and 2) if the
sym link already exists, we want to overwrite it, hence -f.

X-Eudev-Bug: 58
X-Eudev-Bug-URL: https://github.com/gentoo/eudev/issues/58
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>