package/ktap: use kernel-module helper

Passing the KERNEL_SRC variable is no longer needed, since it was only
used to run commands like:
    $(MAKE) -C $(KERNEL_SRC) M=$(PWD) modules

which is basically what the new kernel-module infra is now doing.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>