Add state to string API function

Added a state to string public API function. There was already
a private function (`relay_chn_state_str`) that provides
this functionality. So this function has been renamed to
`relay_chn_state_to_str` and made publicly available.

Refs #1104, #1105 and closes #1108.

src/relay_chn_ctl_single.c

@@ -5,6 +5,7 @@
  */
 
 #include "esp_check.h"
+#include "relay_chn.h"
 #include "relay_chn_priv_types.h"
 #include "relay_chn_core.h"
 #include "relay_chn_ctl.h"
@@ -65,7 +66,7 @@ relay_chn_state_t relay_chn_ctl_get_state()
 
 char *relay_chn_ctl_get_state_str()
 {
-    return relay_chn_state_str(s_chn_ctl.state);
+    return relay_chn_state_to_str(s_chn_ctl.state);
 }
 
 void relay_chn_ctl_run_forward()