bluetooth: samples: Add missing option to hci_uart_async sample

hci_uart_async/debug.mixin.conf is used to enable RTT logging
for hci_uart_async sample. RTT can be used to avoid conflict
between different serial interfaces because the sample uses
UART as an interface for HCI communication.
RTT_CONSOLE option enabled in the debug.mixin.conf depends on
CONFIG_USE_SEGGER_RTT so enabling CONFIG_USE_SEGGER_RTT by
default makes it easier to build the project with RTT enabled

Signed-off-by: Ivan Iushkov <ivan.iushkov@nordicsemi.no>