layer.conf: Don't use indirect help2man-native dependencies

layer.conf: Don't use indirect help2man-native dependencies

Similarly to other tools such as pkgconfig and quilt, don't pull in
help2man-native unless there is a direct DEPENDS. This is generally good
for keeping the recipe sysroots leaner and cleaner and should fix some
issues with squashf-tools in particular.

This will mean any recipe with an indirect dependency will break so
missing recipe dependencies will need to be fixed.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>