gstreamer/gst-plugin-x170: requires the Linux kernel

Add a missing reverse depency from the on2-8170-libs package.

Fixes warning:
warning: (BR2_PACKAGE_GST_PLUGIN_X170) selects BR2_PACKAGE_ON2_8170_LIBS which has unmet direct dependencies (BR2_arm926t && BR2_TOOLCHAIN_USES_GLIBC && BR2_LINUX_KERNEL)

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>