fix pipewire SDL API headers not found

flake.nix

@@ -18,6 +18,11 @@
           inherit system;
           overlays = builtins.attrValues self.overlays;
         }));
+
+    # list directory contents as absolute paths
+    lsAbsolute = with builtins;
+      path:
+        map (name: toPath path + toPath name) (attrNames (readDir path));
   in {
     overlays.default = final: prev: {
       dorne = final.haskellPackages.developPackage {
@@ -42,6 +47,7 @@
 
             # C Dependencies
             pipewire
+
             # Language/Development Tooling
             clang-tools
             bear # clang compile_commands.json
@@ -50,6 +56,11 @@
           ];
 
           shell = "${pkgs.bash}/bin/bash";
+
+          # SPA API is provided by pipewire's "dev" derivation output
+          C_INCLUDE_PATH =
+            builtins.concatStringSep ":"
+            (lsAbsolute "${pkgs.pipewire.dev}/include");
         };
       }
     );