linux/linux.mk: added https:// pattern for kernel patches

Patches located at https:// scheme URL were threated as directories,
causing build failures.

Fixed by adding https:// pattern.

Signed-off-by: Ivo Slanina <ivo.slanina@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>