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

Commit 94c72087f3 ("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>

Config.in.legacy

@@ -123,6 +123,7 @@ config BR2_PACKAGE_MODULE_INIT_TOOLS
 	bool "module-init-tools replaced by kmod"
 	select BR2_PACKAGE_KMOD
 	select BR2_PACKAGE_KMOD_TOOLS
+	select BR2_LEGACY
 	help
 	  The 'module-init-tools' package has been removed, since it
 	  has been depracated upstream and replaced by 'kmod'.