Config.in: switch GNU mirrors URL to https

As explained on [1], the primary GNU mirrors URL is https.
This commit updates BR2_GNU_MIRROR to switch to it.

[1] https://www.gnu.org/prep/ftp.html

Signed-off-by: Julien Olivain <ju.o@free.fr>
Reviewed-by: Vincent Jardin <vjardin@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

Config.in

@@ -285,7 +285,7 @@ config BR2_KERNEL_MIRROR
 
 config BR2_GNU_MIRROR
 	string "GNU Software mirror"
-	default "http://ftpmirror.gnu.org"
+	default "https://ftpmirror.gnu.org"
 	help
 	  GNU has multiple software mirrors scattered around the
 	  world. The following allows you to select your preferred