Add const qualifier to strings we get from Python

nilmdb/server/rocket.c

@@ -350,7 +350,7 @@ static inline long int strtoul10(const char *nptr, char **endptr) {
 static PyObject *Rocket_append_string(Rocket *self, PyObject *args)
 {
 	int count;
-	char *data;
+	const char *data;
 	int offset;
 	int linenum;
 	double start;
@@ -374,7 +374,7 @@ static PyObject *Rocket_append_string(Rocket *self, PyObject *args)
 			      &start, &end, &last_timestamp))
 		return NULL;
 
-	char *buf = &data[offset];
+	const char *buf = &data[offset];
 	while (written < count && *buf)
 	{
 		linenum++;