Fix linker error by making empty_msg static member variable inline constexpr

Author: kzhdev

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Use UTC-aware time conversion (`timegm`/`_mkgmtime`) instead of local time (`mktime`) for ISO 8601 timestamp parsing
 - Replace `std::format` with chrono formatters with `strftime` for GCC 14 compatibility in `timestamp_to_string`
 - Fix `std::string_view` to `std::string` conversion in `logData` for `fstream::open` compatibility
+- Fix linker error by making `empty_msg` static member variable `inline constexpr`
 
 ### Removed
 - Unused `reconnectMarketData` and `reconnectUserData` methods from WebSocket class

include/coinbase/websocket.hpp

@@ -254,7 +254,7 @@ class WebSocketClient {
     std::atomic_int_fast8_t pending_md_socket_close_ = 0;
     std::atomic_int_fast8_t pending_user_socket_close_ = 0;
     uint64_t data_cursor_ = 0;
-    static const char empty_msg = '\0';
+    static inline constexpr char empty_msg = '\0';
 };
 
 constexpr uint32_t MESSAGE_HEADER_SIZE = sizeof(WebSocketClient*) + 1;