helpers: add to_string() for char

Origin commit data
------------------
Commit: 7679168b61
Author: Thilo Graf <dbt@novatux.de>
Date: 2019-05-05 (Sun, 05 May 2019)

src/system/helpers.cpp

@@ -1411,6 +1411,13 @@ std::string to_string(unsigned long long i)
 	s << i;
 	return s.str();
 }
+
+std::string to_string(char i)
+{
+	std::stringstream s;
+	s << i;
+	return s.str();
+}
 #endif
 
 /**