Add vdevprops.7 to the Makefile

Adding vdevprops.7 to the Makefile ensures that it gets installed
properly on FreeBSD.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Daniel Ebdrup Jensen <debdrup@FreeBSD.org>
Closes #14527