[gdbsupport] Ignore pathc in codespell check in gdb_tilde_expand.cc

Ignore the following codespell detections:
...
$ codespell --config gdbsupport/setup.cfg gdbsupport
gdbsupport/gdb_tilde_expand.cc:54: pathc ==> patch
gdbsupport/gdb_tilde_expand.cc:99: pathc ==> patch
...
by adding codespell:ignore comments.