etc/zfs/Makefile.am: set initconfdir

The initconfdir variable is not defined in etc/zfs/Makefile,
so the sed code does not perform the correct replacement.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Issue openzfs#10375 
Closes openzfs#10376

etc/zfs/Makefile.am

@@ -1,4 +1,5 @@
 pkgsysconfdir = $(sysconfdir)/zfs
+initconfdir = $(DEFAULT_INITCONF_DIR)
 
 pkgsysconf_DATA = \
 	vdev_id.conf.alias.example \