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>

ext/pybind11/include/pybind11/pybind11.h

diff --git a/ext/pybind11/include/pybind11/pybind11.h b/ext/pybind11/include/pybind11/pybind11.h
index 613135a..db325e0 100644
--- a/ext/pybind11/include/pybind11/pybind11.h
+++ b/ext/pybind11/include/pybind11/pybind11.h
@@ -317,7 +317,7 @@
             rec->def = new PyMethodDef();
             std::memset(rec->def, 0, sizeof(PyMethodDef));
             rec->def->ml_name = rec->name;
-            rec->def->ml_meth = reinterpret_cast<PyCFunction>(*dispatcher);
+            rec->def->ml_meth = reinterpret_cast<PyCFunction>(reinterpret_cast<void (*) (void)>(*dispatcher));
             rec->def->ml_flags = METH_VARARGS | METH_KEYWORDS;
 
             capsule rec_capsule(rec, [](void *ptr) {