Disable some more leveldb warnings

Summary:
This frees the output from the leveldb warnings.
There is a single -Wmaybe-initialized outside of the tests and it is
relying on the GCC __get_cpuid() function to assign a value to a
pointer, which should be safe.

Test Plan:
With both GCC and Clang:

ninja all check-leveldb

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6285