Initialize const member

[nobu]

  /github/workspace/src/ext/zlib/zlib.c:2608:25: warning: default initialization of an object of type 'struct read_raw_arg' with const member leaves the object uninitialized [-Wdefault-const-init-field-unsafe]
   2608 |     struct read_raw_arg ra;
        |                         ^
  /github/workspace/src/ext/zlib/zlib.c:2450:14: note: member 'argv' declared 'const' here
   2450 |         const VALUE argv[2]; /* for rb_funcallv */
        |                     ^