Bug#871772: grub-mknetdir not enabled for arm64

[Leif Lindholm]

Package: grub2
Version: 2.02~beta3-5

The stretch arm64 build of grub lacks the following commit (which
happened after the -beta3 release):
http://git.savannah.gnu.org/cgit/grub.git/commit/util/grub-mknetdir.c?id=0d663b50b9abf830fd10de384606a0632a605b77

This means that grub-mknetdir fails silently on arm64.

This is already fixed in buster, but would be very nice if it could be
included in future stretch releases.