ext: Upgrade PyBind11 to version 2.2.1This upgrade is necessary for pybind to build with GCC 7.2.We still need to add the patch for stl.h. MSC_FULL_VER change is no longerneeded.See https://gem5-review.googlesource.com/c/public/gem5/+/2230Change-Id: I806729217d022070583994c2dfcaa74476aef30fSigned-off-by: Jason Lowe-Power <jason@lowepower.com>Reviewed-on: https://gem5-review.googlesource.com/5801Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>