rust-hg-core: use correct type for libc hostname buffer
The type of libc::c_char is u8 on aarch64 rather than i8, which causes
the use of a specifically-typed constant to fail.
Differential Revision: https://phab.mercurial-scm.org/D12373
#require docutils gettext $ $TESTDIR/check-gendoc zh_TW checking for parse errors