obs-studio: restrict meta.platforms

Hydra has been trying to build it on aarch64-linux, but never succeeded:
https://hydra.nixos.org/job/nixpkgs/trunk/obs-studio.aarch64-linux/all
(It tries to feed compiler x86-specific options.)
I didn't test i686-linux, due to a transitive dependency not building
(libupnp), but there it might likely work.

pkgs/applications/video/obs-studio/default.nix

@@ -91,6 +91,6 @@ in stdenv.mkDerivation rec {
     homepage = https://obsproject.com;
     maintainers = with maintainers; [ jb55 MP2E ];
     license = licenses.gpl2;
-    platforms = with platforms; linux;
+    platforms = [ "x86_64-linux" "i686-linux" ];
   };
 }