Fix compilation error

src/engine/engine_print.c

@@ -480,7 +480,7 @@ static bool validateFloatFormat(const char* float_format) {
   // flag characters. allow at most one of each flag
   const char flag_characters[] = "-+ #0";
   int flag_character_counts[sizeof(flag_characters)] = { 0 };
-  char* c;
+  const char* c;
   while (c = strchr(flag_characters, float_format[cur_idx]), c != NULL) {
     int flag_idx = (c - flag_characters)/sizeof(char);
     flag_character_counts[flag_idx]++;