samples: bluetooth: Add build asserts for VS-using samples

The README files of these samples already indicate that they use the
Zephyr-specific HCI vendor extensions, but this may still not be clear
enough to some users, as evidenced in #81779. Add build asserts to make
sure the samples only build when Zephyr VS extensions have been enabled.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>