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 de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: