Config.in.legacy: select BR2_LEGACY for module-init-tools legacy option

Commit 94c72087 ("module-init-tools: remove package") removed the
module-init-tools package and therefore added the corresponding config
option in Config.in.legacy. However, the commit forgot to add the
"select BR2_LEGACY" that ensures the build cannot proceed until the
user has acknowledged the change (i.e kmod replacing
module-init-tools).

This commit adds this missing "select BR2_LEGACY".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>