Update the proc-macro2/meson.build to include the changes from v1.0.81.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11071
Signed-off-by: José Expósito <jexposit@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28923>
(cherry picked from commit 18c5315731)