darwin.mDNSResponder: patch header to match SDK

The upstream source release for 765.50.9 defines `_DNS_SD_LIBDISPATCH`
to `0` in `dns_sd.h`, but the SDK version of that header actually
defines it to `1`. Having it defined to `0` causes packages that expect
to use libdispatch-based symbols such as `DNSServiceSetDispatchQueue` to
fail to build. Define it to match the SDK to fix those packages.

pkgs/os-specific/darwin/apple-source-releases/mDNSResponder/default.nix

@@ -4,6 +4,9 @@ appleDerivation' stdenvNoCC {
   dontBuild = true;
   installPhase = ''
     mkdir -p $out/include
-    cp mDNSShared/dns_sd.h $out/include
+    # TODO: Do this only for 765.50.9 once there is a way to apply version-specific
+    # logic in a source-release derivation.
+    substitute mDNSShared/dns_sd.h $out/include/dns_sd.h \
+      --replace '#define _DNS_SD_LIBDISPATCH 0' '#define _DNS_SD_LIBDISPATCH 1'
   '';
 }