Bluetooth: CAP: cap_common.c should only be included with ACL

Since the cap_common.c rely on CONFIG_BT_MAX_CONN, and is
only used for connected procedures, it should be guarded
by CONFIG_BT_CAP_INITIATOR_UNICAST instead of
CONFIG_BT_CAP_INITIATOR

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>