curl: Enable http2Support for darwin

http2Support was disabled due to a bootstrapping issue involving xz.
Now that xz is available in the bootstrap environment for all
platforms, http2Support can be enabled globally.

pkgs/top-level/all-packages.nix

@@ -1719,7 +1719,7 @@ with pkgs;
 
   curl = callPackage ../tools/networking/curl rec {
     fetchurl = fetchurlBoot;
-    http2Support = !stdenv.isDarwin;
+    http2Support = true;
     zlibSupport = true;
     sslSupport = zlibSupport;
     scpSupport = zlibSupport && !stdenv.isSunOS && !stdenv.isCygwin;