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>

samples/bluetooth/hci_uart_async/debug.mixin.conf

@@ -4,6 +4,8 @@ CONFIG_DEBUG_INFO=y
 CONFIG_DEBUG_OPTIMIZATIONS=y
 CONFIG_DEBUG_THREAD_INFO=y
 
+# Enable RTT
+CONFIG_USE_SEGGER_RTT=y
 # Enable RTT console
 CONFIG_RTT_CONSOLE=y