opusfile: add "dev" output

`opusfile` is a a popular game dependency via `SDL2_mixer`. Split it's
`dev` output to separate store path to reduce runtime closure size.

While at it amend games's include paths.

pkgs/applications/audio/opusfile/default.nix

···
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ openssl libogg ];
   propagatedBuildInputs = [ libopus ];
+  outputs = [ "out" "dev" ];
   patches = [ ./include-multistream.patch ]
     # fixes problem with openssl 1.1 dependency
     # see https://github.com/xiph/opusfile/issues/13

pkgs/games/iortcw/sp.nix

···
 
   NIX_CFLAGS_COMPILE = [
     "-I${SDL2.dev}/include/SDL2"
-    "-I${opusfile}/include/opus"
+    "-I${opusfile.dev}/include/opus"
   ];
   NIX_CFLAGS_LINK = [ "-lSDL2" ];