Turn off -Wmaybe-uninitialized

Turn off -Wmaybe-uninitialized

In gcc, maybe-uninitialized gives different warnings depending on the
optimization level, and in our experience usually gives false
positives.  We don't ask for it (except implicitly through -Wall), but
gcc bundles it into the error behavior of -Werror=uninitialized.
Explicitly turn it off so that builds with -Og and -Os don't error
out.