pxlib: expand platforms to all

pkgs/development/libraries/pxlib/default.nix

···
     description = "Library to read and write Paradox files";
     homepage = "http://pxlib.sourceforge.net/";
     license = licenses.gpl2;
-    platforms = platforms.linux;
+    platforms = platforms.all;
     maintainers = [ maintainers.winpat ];
   };
 }