libfetch: unquote @LIBFETCH_SONAME@ subst

@LIBFETCH_SONAME@ is no longer quoted. The C define still is.

Ref: 153f7c9f72082d7ef5ee27fcbec1bcb94ba88151
Ref: https://github.com/openzfs/zfs/pull/12835#discussion_r776833743
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12922