Bluetooth: tests: shell: Fix build error

This fixes build error that happens when shell is built without CAP
Acceptor Set Member support. The cap_acceptor.c contains shell commands
for Set Member only, thus it's OK to build the code if
CONFIG_BT_CAP_ACCEPTOR_SET_MEMBER is set for now. This can be modified
in the future when shell will be extended with other
Set Member non-specific CAP commands.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>