Makefile: fix install_udev target by s/TARGETS/MAKECMDGOALS/

Makefile: fix install_udev target by s/TARGETS/MAKECMDGOALS/

A Gentoo user reported [1] that the 'install_udev' target would
install a systemd-flavored udev rules, despite the 'systemd' target
not being selected. Replacing 'TARGETS' with 'MAKECMDGOALS' in the
Makefile rule fixes this .

I am not sure where the TARGETS variable origniates from, but GNU make
uses MAKECMDGOALS (FreeBSD make uses .TARGETS, i.e., with a leading
dot). So if further portability between make variants is required,
then this needs to be adjusted.

1: https://bugs.gentoo.org/803545#c3
Fixes: ae1f18a06039 ("udev rule: Dynamically call either systemd or autorandr directly depending on whether systemd unit is available")