Fixed bootstrapping bug with upb when building from a different repo.

This is essentially backporting a fix that exists on main already:
https://github.com/protocolbuffers/protobuf/blob/a3c33a87c1f24a7aa082317257a9ee297b4d883d/upb_generator/bootstrap_compiler.bzl#L23

Author: haberman

build_defs/upb.patch

@@ -5,7 +5,7 @@
  # begin:github_only
  _is_google3 = False
 -_extra_proto_path = "-Iexternal/com_google_protobuf/src "
-+_extra_proto_path = "-Isrc "
++_extra_proto_path = "-I$$(dirname $(location @com_google_protobuf//:descriptor_proto_srcs))/../.. "
  # end:github_only
 
  def _upbc(stage):