syslog:syslog_channel:Fix compiler warnings

syslog_channel.c:98:8: error: unknown type name 'sem_t'
   syslog/syslog_channel.c:99:14: error: 'g_syslog_default_sem'
   defined but not used

drivers/syslog/syslog_channel.c

@@ -27,6 +27,7 @@
 #include <sys/types.h>
 #include <assert.h>
 #include <errno.h>
+#include <semaphore.h>
 
 #include <nuttx/syslog/syslog.h>
 #include <nuttx/compiler.h>
@@ -95,7 +96,9 @@ static struct syslog_channel_s g_rpmsg_channel =
 #endif
 
 #if defined(CONFIG_SYSLOG_DEFAULT)
+#  if defined(CONFIG_ARCH_LOWPUTC)
 static sem_t g_syslog_default_sem = SEM_INITIALIZER(1);
+#  endif
 
 static const struct syslog_channel_ops_s g_default_channel_ops =
 {