media: v4l2-pci-skeleton: depend on CONFIG_SAMPLES

Commit 0185f850 ("[media] samples: v4l: from Documentation to
samples directory") moved the v4l2-pci-skeleton driver to the samples
directory. The samples are only be built, if CONFIG_SAMPLES is enabled.

Therefore, VIDEO_PCI_SKELETON is not enough to build the
v4l2-pci-skeleton driver, but SAMPLES needs to be enabled, too. Let
VIDEO_PCI_SKELETON depend on SAMPLES.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>