Avoid warning about multiple outputs when building private directory name
Avoid warning about multiple outputs when building private directory name

Fixup for b4b1a2c5a145c1459fc4563a289e164e23bd6a02.

A warning would be printed for any rule with multiple outputs, for
example:
WARNING: custom_target 'coredump.conf.5' has more than one output! Using the first one.
WARNING: custom_target 'dnssec-trust-anchors.d.5' has more than one output! Using the first one.
WARNING: custom_target 'halt.8' has more than one output! Using the first one.

Fixes https://github.com/systemd/systemd/issues/16461.

QuLogic/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,pexip/meson,QuLogic/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson