libselinux: fix build with musl (#119472)

Co-authored-by: Kasper Gałkowski <kpg@posteo.net>
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/os-specific/linux/libselinux/default.nix

@@ -44,6 +44,11 @@ stdenv.mkDerivation rec {
     "PYTHONLIBDIR=$(py)/${python3.sitePackages}"
   ];
 
+  postPatch = lib.optionalString stdenv.hostPlatform.isMusl ''
+    substituteInPlace src/procattr.c \
+      --replace "#include <unistd.h>" ""
+  '';
+
   preInstall = optionalString enablePython ''
     mkdir -p $py/${python3.sitePackages}/selinux
   '';