luci(-ssl)-nginx: depend on luci-app-opkg

Commit 391e82d6

 in 2018 added luci-app-opkg dependency to
the 'luci' collection and its derivatives, but as nginx collections
do not depend on main 'luci', they did not get the intended change.

Fixes #4148

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>