Fix build issues

[NikolaiShipilov]

Addresses issues identified during the development of https://github.com/openDAQ/openDAQ_LT_reference_example :

• Adds missing definitions
• Fixes WEBSOCKET_STREAMING macro usage by using 1/0 instead of relying on its presence as a definition
• Adds missing system includes
• Includes "streaming_config.h" as the first header in files where configuration parameters are used
• Fixes variable declaration order and adds missing type specifiers
• Removes the redundant declaration of the static signals_add_signal function