explicit sign on result as chars are implicitly unsigned on riscv

sorting.c

@@ -38,7 +38,7 @@ int sort_natural(const void *file0, const void *file1){
 	long parsed_number0 = 0;
 	long parsed_number1 = 0;
 	char is_num = 0;
-	char result = 0;
+	signed char result = 0;
 
 	do {