webrtc-audio-processing: split headers to "dev" output

pkgs/development/libraries/webrtc-audio-processing/0.3.nix

···
     sha256 = "1gsx7k77blfy171b6g3m0k0s0072v6jcawhmx1kjs9w5zlwdkzd0";
   };
 
+  outputs = [ "out" "dev" ];
+
   patches = [
     ./enable-riscv.patch
     ./enable-powerpc.patch