antlr4_x.runtime.cpp: fix build error due to sourceRoot missing source prefix

Fixes https://github.com/NixOS/nixpkgs/issues/197985

Patch from @NickCao https://github.com/NixOS/nixpkgs/issues/197985#issuecomment-1292843789

pkgs/development/tools/parsing/antlr/4.nix

···
         pname = "antlr-runtime-cpp";
         inherit version;
         src = source;
-        sourceRoot = "runtime/Cpp";
+        sourceRoot = "source/runtime/Cpp";
 
         outputs = [ "out" "dev" "doc" ];