npth: add musl regression test

Would have caught the issue fixed by
180b5c1 ("pkgsMusl.npth: backport patch to fix build").

I used pkgsCross instead of pkgsMusl so it does something useful on
non-Linux.

pkgs/development/libraries/npth/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, fetchpatch, autoreconfHook }:
+{ lib, stdenv, fetchurl, fetchpatch, autoreconfHook, pkgsCross }:
 
 stdenv.mkDerivation rec {
   pname = "npth";
@@ -21,6 +21,10 @@ stdenv.mkDerivation rec {
 
   doCheck = true;
 
+  passthru.tests = {
+    musl = pkgsCross.musl64.npth;
+  };
+
   meta = with lib; {
     description = "The New GNU Portable Threads Library";
     mainProgram = "npth-config";