shells/fish: link against devel/ncurses if it's installed at build time

shells/fish is built using CMake and requires the FindCurses module. Unless
CURSES_NEED_NCURSES=TRUE, shells/fish will always be built against ncurses
from base, even when devel/ncurses is already installed.

This change has no effect for packages built from Poudriere, which will not
install devel/ncurses when building fish.

PR: 246365
Submitted by: Dustin Boyd <memreflect@pm.me>