Remove incorrect use of EXTRA_DIST for man pages

Setting the 'dist_' prefix is the correct way to instruct Automake
to include these files in the distribution. The EXTRA_DIST variable
is reserved for files which are not covered by the automatic rules.

http://www.gnu.org/software/automake/manual/automake.html#Basics

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>