syslog/rpmsg: Fix warning: ‘syslog_rpmsg_write’ defined but not used

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

include/nuttx/syslog/syslog_rpmsg.h

@@ -59,7 +59,10 @@ extern "C"
 void syslog_rpmsg_init_early(FAR const char *cpu_name, FAR void *buffer,
                              size_t size);
 int syslog_rpmsg_init(void);
+
 int syslog_rpmsg_putc(int ch);
+int syslog_rpmsg_flush(void);
+ssize_t syslog_rpmsg_write(FAR const char *buffer, size_t buflen);
 #endif
 
 #ifdef CONFIG_SYSLOG_RPMSG_SERVER