Update test/test_buffer.h

[maflcko]

This fixes a compile error on clang-18:

In file included from src/fuzz/generate_corpus.c:13: src/fuzz/../test/test_buffer.h:13:25: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
   13 | rt_buffer_t *buffer_init();
      |                         ^
      |                          void
1 error generated.