fig2dev: Set platforms to unix

pkgs/applications/graphics/fig2dev/default.nix

···
     description = "Tool to convert Xfig files to other formats";
     homepage = "http://mcj.sourceforge.net/";
     license = licenses.xfig;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ lesuisse ];
   };
 }