fswebcam: disable static build

fswebcam doesn't play nicely with static build as it doesn't track its
indirect dependencies in link time. Fixing this requires tracking all indirect
optional dependencies, which is tedious and error prone. Just disable static
build.

This build failure didn't show on the autobuilder for some reason.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>