Fix: remove platform declaration, add eventfd

"if platforms is not set it will default to go.meta.platforms which is probably
what you want" And that does sound like what we want. Also, the linux build was
complaining about eventfd, or the lack thereof.

pkgs/servers/holochain-go/default.nix

@@ -24,7 +24,6 @@ buildGoPackage rec {
     downloadPage = "https://developer.holochain.org/";
     license = stdenv.lib.licenses.gpl3;
     maintainers = with stdenv.lib.maintainers; [ gavin ];
-    platforms = stdenv.lib.platforms.all;
 
   };
 }

pkgs/servers/holochain-go/deps.nix

@@ -171,6 +171,15 @@
       sha256 = "1q23y4lacsz46k9gmgfw4iwwydw36j2601rbidmmswl94grpc386";
     };
   }
+  {
+    goPackagePath = "github.com/gxed/eventfd";
+    fetch = {
+      type = "git";
+      url = "https://github.com/gxed/eventfd";
+      rev = "80a92cca79a8041496ccc9dd773fcb52a57ec6f9";
+      sha256 = "1p15rsimkcp2flj6wb41flac72zhf97zd8jxrahsw05nyfbp58z5";
+    };
+  }
   {
     goPackagePath = "github.com/huin/goupnp";
     fetch = {