This allows only installing the data files in case the build is done
separately (like for example when using debcargo/dh-cargo which uses
rustc --target option and thus the speakersafetyd ends up in a different
path which includes the target).
While at it also mark all targets as PHONY, since none of them are
backed by an actual file with the same name.
Signed-off-by: Andreas Henriksson <andreas@fatal.se>