samtragar has answered the question, but to be able to discover this for yourself, you'll need to add some options to the build process so that the intermediate files are retained and you can inspect them. Something like this is useful:

use Inline 'NoClean', 'FORCE', 'INFO' ;
use Inline Config => WARNINGS => 4;
use Inline 'C' => <<CEND, NAME =>'yourname';
...
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Note: I've also attempted to enable warnings above. It doesn't work, or at least they are not displayed when the program is built. If they were, the C compiler would have warned that you were assigning a SV** to an SV*.

The only way I have found to get the warning displayed on my system is to deliberately include a fatal error. A line like fred; will give an error

test.xs(13) : error C2065: 'fred' : undeclared identifier
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And the warnings will also be displayed.

There has to be a better way, but I haven't found it.