tests: fix test failures on upcoming gcc-14 (#671)

Upcoming `gcc-14` enabled a few warnings into errors, like
`-Wint-conversion`. This caused `shadow` build to fail as:

    src/extended/extended.c:13:9: error:
      initialization of ‘int’ from ‘void *’ makes integer from pointer
        without a cast [-Wint-conversion]
       13 |         NULL,
          |         ^~~~

The change fixes `int ml_flags;` to take `int` instead of pointer.

tests/masonry/builders/fixtures/extended/extended/extended.c

@@ -10,7 +10,7 @@ static PyMethodDef module_methods[] = {
     {
         "hello",
         (PyCFunction) hello,
-        NULL,
+        0,
         PyDoc_STR("Say hello.")
     },
     {NULL}

tests/masonry/builders/fixtures/extended_with_no_setup/extended/extended.c

@@ -10,7 +10,7 @@ static PyMethodDef module_methods[] = {
     {
         "hello",
         (PyCFunction) hello,
-        NULL,
+        0,
         PyDoc_STR("Say hello.")
     },
     {NULL}

tests/masonry/builders/fixtures/src_extended/src/extended/extended.c

@@ -10,7 +10,7 @@ static PyMethodDef module_methods[] = {
     {
         "hello",
         (PyCFunction) hello,
-        NULL,
+        0,
         PyDoc_STR("Say hello.")
     },
     {NULL}