man/Makefile.am: put zfs_prepare_disk.8 into substitution list.

b53077a forgot to include the zfs_prepare_disk.8 in the
substitution list, although it built in the past because 38e2e9c
accidentally dropped the $(SUBSTFILES) from the "%: %.in" pattern.

Since we recovered the scope of $(SUBSTFILES) to explicitly control
what files are being generated from the corresponding .in, this fix is
needed.

Signed-off-by: Benda Xu <orv@debian.org>

man/Makefile.am

@@ -119,7 +119,7 @@ nodist_man_MANS = \
 
 dist_noinst_DATA += $(dist_noinst_man_MANS) $(dist_man_MANS)
 
-SUBSTFILES += $(nodist_man_MANS)
+SUBSTFILES += $(nodist_man_MANS) %D%/man8/zfs_prepare_disk.8
 
 CHECKS += mancheck
 mancheck: