Config.in.legacy: add missing select

BR2_LINUX_KERNEL_CUSTOM_LOCAL was moved to legacy handling during release
2016.11 but does not select BR2_LEGACY.
Add the select, like done by all other symbols in the file.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Config.in.legacy

@@ -3953,6 +3953,7 @@ config BR2_PACKAGE_ELEMENTARY
 
 config BR2_LINUX_KERNEL_CUSTOM_LOCAL
 	bool "Linux kernel local directory option removed"
+	select BR2_LEGACY
 	help
 	  The option to select a local directory as the source of the
 	  Linux kernel has been removed. It hurts reproducibility of