soxr: fix include directory in pkgconfig file

The include directory in soxr.pc points to the wrong output. Fix it.

(Upstream seems dead -- the last upstream commit is from 2018 and the
few merge requests that have been made since then are neither merged nor
closed.)

Fixes: a7853ab93a06d5fe ("soxr: extract dev output")

Changed files
+5
pkgs
by-name
so
+5
pkgs/by-name/so/soxr/package.nix
···
nativeBuildInputs = [ cmake ];
meta = with lib; {
description = "Audio resampling library";
homepage = "https://soxr.sourceforge.net";
···
nativeBuildInputs = [ cmake ];
+
cmakeFlags = [
+
# Workaround for upstream not using GNUInstallDirs.
+
"-DINCLUDE_INSTALL_DIR=${placeholder "dev"}/include"
+
];
+
meta = with lib; {
description = "Audio resampling library";
homepage = "https://soxr.sourceforge.net";