gnupg: drop mandatory dependency on ncurses

This package only needs ncurses when readline support is enabled, as
it's the autoconf macro file for readline (used by autoconf to create
the gnupg configure script) that checks for and pulls in ncurses.

Since readline already depends on ncurses, gnupg need only depend on
readline (when enabled).

The host package always forces readline support off, so the
host-ncurses dependency can be removed entirely.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>