Update multibuild to a version that strips binaries in wheels

[xhochy]

On the next release, please update the multibuild submodule to matthew-brett/multibuild@b2748e5 or newer. This should reduce the size of the Pandas wheels from 27M to 11M by excluding unnecessary debug symbols.

ref pandas-dev/pandas#19531

[jorisvandenbossche]

I think we can already do that now? (to make sure we don't forget it)