Add ___tracy_init_thread function

This function needs to be called for each non-main thread before calling
the `___tracy_alloc_` functions.

Alternative way to achieve this could be initializing the allocator
transparently in the `___tracy_alloc_*` calls.

TracyC.h

@@ -81,6 +81,7 @@ struct ___tracy_c_zone_context
 // This struct, as visible to user, is immutable, so treat it as if const was declared here.
 typedef /*const*/ struct ___tracy_c_zone_context TracyCZoneCtx;
 
+TRACY_API void ___tracy_init_thread(void);
 TRACY_API uint64_t ___tracy_alloc_srcloc( uint32_t line, const char* source, const char* function );
 TRACY_API uint64_t ___tracy_alloc_srcloc_name( uint32_t line, const char* source, const char* function, const char* name, size_t nameSz );