lowdown: fix musl build
lowdown: fix musl build

Whenever building a shared library, we need to compile with -fPIC.
(This is sometimes implicitly added by the hardening flags, but not
always, e.g. for pkgsMusl.)

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs