ext: Cherry-pick pybind change to support GCC 8

ext: Cherry-pick pybind change to support GCC 8

GCC 8's -Wcast-function-type didn't like this line in pybind. Rather
than updating pybind to the HEAD, we'll wait for a new release. The last
release was in April and didn't include this change.

See https://github.com/pybind/pybind11/pull/1396.

Change-Id: I199c3023487d5a73a745396df8e7fcd6635a146c
Reported-by: Thawra Kadeed <kadeed@ida.ing.tu-bs.de>
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/11909
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>