shairport-sync: add xxd to the nativeBuildInputs

it is run while building shairport-sync and strictdeps means that it is
not added to the path in buildInputs

pkgs/servers/shairport-sync/default.nix

@@ -58,6 +58,8 @@ stdenv.mkDerivation rec {
     # To achieve this, we coerce the output to a string to prevent
     # mkDerivation's splicing logic from kicking in.
     "${glib.dev}"
+  ] ++ optional enableAirplay2 [
+    unixtools.xxd
   ];
 
   makeFlags = [
@@ -83,7 +85,6 @@ stdenv.mkDerivation rec {
     libgcrypt
     libuuid
     ffmpeg
-    unixtools.xxd
   ]
   ++ optional stdenv.isLinux glib;