qrupdate: Add which to native build inputs

The build won't fail without it, but it's needed according to:
https://sourceforge.net/p/qrupdate/code/HEAD/tree/test/report_results

pkgs/development/libraries/qrupdate/default.nix

@@ -3,6 +3,7 @@
 , gfortran
 , blas
 , lapack
+, which
 }:
 stdenv.mkDerivation {
   name = "qrupdate-1.1.2";
@@ -33,6 +34,8 @@ stdenv.mkDerivation {
 
   buildInputs = [ gfortran blas lapack ];
 
+  nativeBuildInputs = [ which ];
+
   meta = with stdenv.lib; {
     description = "Library for fast updating of qr and cholesky decompositions";
     homepage = "https://sourceforge.net/projects/qrupdate/";