rpmsg_uart: fix compile break when enable rptun

N/A

nuttx.rel: In function `rpmsg_serialinit':
nuttx/arch/sim/src/sim/up_rptun.c:257: undefined reference to `uart_rpmsg_init'
collect2: error: ld returned 1 exit status
Makefile:310: recipe for target 'nuttx' failed

Change-Id: I93a20941bc07f749165dc8f012da46ddb7b02b00
Signed-off-by: dongjiuzhu <dongjiuzhu1@xiaomi.com>

arch/sim/src/sim/up_rptun.c

@@ -248,6 +248,7 @@ int up_rptun_init(void)
   return 0;
 }
 
+#if CONFIG_RPMSG_UART
 void rpmsg_serialinit(void)
 {
 #if CONFIG_SIM_RPTUN_MASTER
@@ -256,3 +257,4 @@ void rpmsg_serialinit(void)
   uart_rpmsg_init("server", "proxy", 4096, true);
 #endif
 }
+#endif