Fix unused variable warning

TAG constant is only used when run limit is enabled. As a result,
the compiler generates an "unused variable" warning for other cases.
Fixed this warning by adding the unused attribute.

src/relay_chn_ctl_single.c

@@ -15,7 +15,7 @@
 #include "relay_chn_nvs.h"
 #endif
 
-static const char *TAG = "RELAY_CHN_CTL";
+static const char *TAG __attribute__((unused)) = "RELAY_CHN_CTL";
 
 static relay_chn_ctl_t s_chn_ctl;