Correct type of loop variable

If `-Wpedantic` is given as part of CFLAGS, this flags a warning stating
that `int` and `size_t` are of different sizes. This simply changes the
type of `i` to `size_t` to match the output of `strlen`.

utils/test/x52_test.c

@@ -160,7 +160,7 @@ static int run_tests(int test_set)
 }
 
 void underline(const char *msg) {
-    int i;
+    size_t i;
     puts(msg);
     for (i = 0; i < strlen(msg); i++) {
         putchar('=');