Fix enconding of addendum

There is a problem with the addendum output, which does show wrong
characters.  I don't know if that problem was there before (broken
addenda are usual). I set opt_es:"-L ISO-8859-1 -A utf-8" to make it
work. Sometimes I go crazy with encodings (the terminal, vim, etc)

man/po/po4a.cfg

@@ -4,7 +4,7 @@
 # Make sure the man pages are generated with the right encoding.
 [po4a_alias:man] man                                      \
                  opt_de:"-L ISO-8859-1"                   \
-                 opt_es:"-L ISO-8859-1"                   \
+                 opt_es:"-L ISO-8859-1"    -A utf-8       \
                  opt_fr:"-L ISO-8859-1"                   \
                  opt_hu:"-L ISO-8859-2     -A ISO-8859-2" \
                  opt_ja:"-L euc-jp         -A utf-8"      \