utest: serial_v2: add depends on RT_USING_SERIAL_V2

Add dependent of RT_USING_SERIAL_V2 in Kconfig to
make sure utest for serial_v2 is only available
when RT_USING_SERIAL_V2 is enabled.

Otherwise menuconfig allow people enable UTEST_SERIAL_TC
but building of utestcases failed. It looks weird and
better to be fixed.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

examples/utest/testcases/drivers/serial_v2/Kconfig

@@ -3,6 +3,7 @@ menu "Utest Serial Testcase"
     config UTEST_SERIAL_TC
         bool "Serial testcase"
         default n
+        depends on RT_USING_SERIAL_V2
 
     if UTEST_SERIAL_TC