Update cmetrics_decode_fuzz.c: Add missing include

[maflcko]

Add missing include to fix clang-18 error:

2 warnings and 1 error generated.
/src/fluent-bit/tests/internal/fuzzers/cmetrics_decode_fuzz.c:53:18: error: call to undeclared function 'cmt_decode_msgpack_create'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   53 |         result = cmt_decode_msgpack_create(&cmt, (char *) data, size, &off);
      |                  ^

---

Testing

I've only tested this via oss-fuzz, after bumping the oss-fuzz clang version to clang-18.

I've used this command in the oss-fuzz project to test:

oss-fuzz# python3 infra/helper.py build_fuzzers --sanitizer address    --engine libfuzzer fluent-bit

Docs, Backport, or other testing should not be needed, I think.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[edsiper]

thank you