elm: Add hack to make test case work

elm: Add hack to make test case work

Because of the way elm_code test case is written, by directly including
the C file, we end up with two symbols for the internal _elm_legacy_add
flag. This makes some test case fail (after applying a pending patch).

Solution found by Sungtaek Hong.

elm_code test case needs to be fixed. Don't include the C files
directly, testing static inlines should be done through a common .x or
something, but not by including the C file itself. This has led and will
lead to many issues.