linux-postmarketos-qcom-msm8916: enable CONFIG_USB_VIDEO_CLASS

Diff is a little noisier than I'd have preferred (soz), but this is what you get if you run pmbootstrap kconfig edit linux-postmarketos-qcom-msm8916 --arch aarch64 and menuconfig your way to victory 😄

I just enabled the basic CONFIG_USB_VIDEO_CLASS, which ensures uvcvideo.ko module shows up (currently doesn't, AFAICT). I've then tested this change with a samsung-a5 successfully showing a USB HDMI dongle + webcam (via OTG adapter) in Cheese/ffplay.