syslog_intbuffer:fix build warning

CC:  mqueue/msgsnd.c syslog/syslog_intbuffer.c: In function 'syslog_flush_internal':
syslog/syslog_intbuffer.c:99:14: warning: unused variable 'flags' [-Wunused-variable]
   99 |   irqstate_t flags;
      |              ^~~~~

Signed-off-by: chenrun1 <chenrun1@xiaomi.com>

Author: crafcat7

drivers/syslog/syslog_intbuffer.c

@@ -96,7 +96,6 @@ static struct syslog_intbuffer_s g_syslog_intbuffer =
 
 static void syslog_flush_internal(bool force, size_t buflen)
 {
-  irqstate_t flags;
   FAR char *buffer;
   size_t size;