cmake: enable warnings for missing function declarations

Over time, we have accumulated quite a lot of functions with missing
prototypes, missing `static` keywords or which were completely unused.
It's easy to miss these mistakes, but luckily GCC and Clang both have
the `-Wmissing-declarations` warning. Enabling this will cause them to
emit warnings for every not-static function that doesn't have a previous
declaration. This is a very sane thing to enable, and with the preceding
commits all these new warnings have been fixed.

So let's always enable this warning so we won't introduce new instances
of them.

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