rootfs: install the AYANEO 3 udev rule in the install target

[matmartinez]

rootfs/usr/lib/udev/rules.d/70-ayaneo-modules.rules has been in the tree
since #536, but rootfs/Makefile's install target never installs it, so
make install doesn't place it under $(PREFIX)/lib/udev/rules.d/. Every
other file in rootfs/ is installed by that target; this one was missed.

The practical effect: distro packages built via make install (Bazzite's
Terra package among them) ship OpenGamepadUI without the rule, so the AYANEO
3 plugin's eject / reset / controller_power sysfs attributes stay
root-only and the plugin can't perform a module eject without the user
dropping the rule into /etc/udev/rules.d/ by hand. With the rule installed
(to /usr/lib/udev/rules.d/ when PREFIX=/usr) eject works out of the box.

This adds the rule to the install target (and removes it in uninstall),
following the same install -Dm644 … $(PREFIX)/lib/… pattern as the systemd
user units directly above it.

Verified that the install line lands the rule at
$(PREFIX)/lib/udev/rules.d/70-ayaneo-modules.rules (i.e.
/usr/lib/udev/rules.d/… for a system install).

Note for packagers: the file now reaches the build root, but a spec/recipe
that enumerates %files explicitly (rather than globbing the install tree)
will also need to list the new path to include it in the package.

[ShadowApex]

The RPM build is currently failing with this change:
https://github.com/ShadowBlip/OpenGamepadUI/actions/runs/36744549618/job/111245124550#step:3:10071

Once you update the RPM spec with this new file, the build should pass.

[matmartinez]

Thanks — fixed in 83b67b8. The spec's %files enumerates paths explicitly, so the rule that make install now drops under /usr/lib/udev/rules.d/ was landing unpackaged; I've added it to the list right after the systemd user units. This is the packager case I'd flagged in the PR description, now handled in-tree. CI should be green on the next run.