apr-util: Fixup apu-1-config

apu-1-config --ldflags is giving broken output on the fixed-output
dependencies. This broke serf build since db was split to multiple
outputs.

https://nix-cache.s3.amazonaws.com/log/q35s5jba6p6dpdqlfn9qgy5w0hpa506v-serf-1.3.9.drv