[cli] fix RTT compiling issue (#9385)

If macro OPENTHREAD_UART_RTT_ENABLE is not enabled, the file
SEGGER_RTT.h and related should not be included.

examples/platforms/utils/uart_rtt.c

@@ -40,12 +40,12 @@
 #include <utils/code_utils.h>
 #include <openthread/error.h>
 
+#if OPENTHREAD_UART_RTT_ENABLE
+
 #include "SEGGER_RTT.h"
 #include "uart.h"
 #include "uart_rtt.h"
 
-#if OPENTHREAD_UART_RTT_ENABLE
-
 static bool sUartInitialized = false;
 static bool sUartPendingUp   = false;