Makefile.inc1: Fix distributeworld mtree mangling for dist root dir

The trailing slash means that ./base itself doesn't get mangled and
remains as-is in the output, leading to a stray /base in base.txz for
NO_ROOT builds and thus in the installed system. Since this action is
running on a line whose file matches one listed by find (and we're
printing all of these as part of that distribution), we don't need to
care about the possibility of a path like ./basefoo/bar where the path
prefix isn't ./base, and can thus just drop the slash rather than
needing something more complicated like "slash or whitespace or EOL" as
one might first think.