discord-gamesdk: install header file

This installs `c/discord_game_sdk.h` into $dev.
This also adds a new output for this header file.

pkgs/development/libraries/discord-gamesdk/default.nix

@@ -14,9 +14,11 @@ stdenv.mkDerivation rec {
     stripRoot = false;
   };
 
-  nativeBuildInputs = [
-    autoPatchelfHook
-  ];
+  outputs = [ "out" "dev" ];
+
+  buildInputs = [ (stdenv.cc.cc.libgcc or null) ];
+
+  nativeBuildInputs = [ autoPatchelfHook ];
 
   installPhase =
     let
@@ -28,6 +30,8 @@ stdenv.mkDerivation rec {
 
       install -Dm555 lib/${processor}/discord_game_sdk${sharedLibrary} $out/lib/discord_game_sdk${sharedLibrary}
 
+      install -Dm444 c/discord_game_sdk.h $dev/lib/include/discord_game_sdk.h
+
       runHook postInstall
     '';