[Feat] Add xxhash dependency for fast hashing in prefix caching

Author: LuminolT

requirements/common.txt

@@ -46,6 +46,7 @@ scipy # Required for phi-4-multimodal-instruct
 ninja # Required for xgrammar, rocm, tpu, xpu
 pybase64 # fast base64 implementation
 cbor2 # Required for cross-language serialization of hashable objects
+xxhash # Required for fast hashing for prefix caching
 setproctitle # Used to set process names for better debugging and monitoring
 openai-harmony >= 0.0.3  # Required for gpt-oss
 anthropic == 0.71.0