package/rpi-userland: fix fall-out after cmake infra change

Cset 9fa7f2b1 (pkg-cmake.mk: build shared library when !BR2_PREFER_STATIC_LIB)
broke rpi-userland.

Force not building shared libs for rpi-userland, since they are broken.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>