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>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/zlib/trees.c

@@ -237,7 +237,7 @@ local void send_bits(s, value, length)
 /* ===========================================================================
  * Initialize the various 'constant' tables.
  */
-local void tr_static_init()
+local void tr_static_init(void)
 {
 #if defined(GEN_TREES_H) || !defined(STDC)
     static int static_init_done = 0;