zlib: trees.c: Fix a warning with clang-15

With clang-15 we now will get warnings such as:

warning: a function declaration without a prototype is deprecated in all
versions of C [-Wstrict-prototypes]

And it is easy enough to address this warning here, even if we would
like to stay in sync more with upstream as it's a single location.

Signed-off-by: Tom Rini <trini@konsulko.com>