v4l2-pci-skeleton: Only build if PCI is available

Currently arm64 does not support PCI but it does support v4l2. Since the
PCI skeleton driver is built unconditionally as a module with no dependency
on PCI this causes build failures for arm64 allmodconfig. Fix this by
defining a symbol VIDEO_PCI_SKELETON for the skeleton and conditionalising
the build on that.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org> [added VIDEO dependencies]
Signed-off-by: Jiri Kosina <jkosina@suse.cz>