initramfs: add gen_init_cpio to hostprogs unconditionally

initramfs: add gen_init_cpio to hostprogs unconditionally

gen_init_cpio is currently only needed when an initramfs cpio archive is
to be created out of CONFIG_INITRAMFS_SOURCE's contents. In other cases,
it's not added to hostprogs and no make target is available.

In preparation to use the host program from Makefile.package, define it
unconditionally. The program will still only be built as needed.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251125-cpio-modules-pkg-v2-1-aa8277d89682@pengutronix.de
Signed-off-by: Nicolas Schier <nsc@kernel.org>