openssh: -fno-common fix from upstream f47d72ddad

This is currently staged in vendor/ as part of the 8.0p1 import, which isn't
quite ready to land. Given that this is a simple one-line fix, apply it now
as the fallout will be pretty minimal.

-fno-common will become the default in GCC10/LLVM11.

MFC after: 3 days