systemd: add missing dependency on libkmod to udev

As of systemd 256, libsystemd-shared.so doesn't directly link to a number
of libraries but instead dlopen()'s them as needed to reduce the size of
the attack surface.

Instead the .so has a .note.dlopen segment that lists the libraries that
may be opened, with the intention that these are transformed into package
recommendation fields.

We don't yet have support for these (see #15595) so explicit dependencies
have been added to the systemd package itself. However, in an initramfs
with udev but without systemd and no recommendations you end up without
libkmod, so module loading is impossible.

Add an explicit hard dependency on libkmod to udev, because modules are
critical functionality.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>