clang-format: add statement macro

Disabled for now but corrects syntax when we switch to v8.

.clang-format

@@ -224,4 +224,5 @@ ForEachMacros:
 # Use once we bump the minimum verison to version 8.
 # # Without this string literals that in-line 'STRINGIFY' behave strangely (a bug?).
 # StatementMacros:
+#   - PyObject_VAR_HEAD
 #   - STRINGIFY