Only build smat.go when fuzzing

There's a `init` function defined in this file, so it always gets
executed, even if the library is not being fuzzed. Mark the file to only
be build during fuzzing. This avoids some compiled code and init work
being done. My primary motivation for this change is that it reduces the
compiled size of this library by ~240KiB.

I'm not able to confirm smat fuzzing still works. Before and after the
change, following the steps in the README.md doesn't result in any
fuzzing.