Adding missing include for CMake FetchContent_Declare command

[grabusr]

In main CMakeLists.txt file if nlohmann_json library is not found in system then is fetched from github repository by using FetchContent_Declare command which should be included from FetchContent_Declare module. Without that include there is CMake error:

CMake Error at CMakeLists.txt:22 (FetchContent_Declare):
  Unknown CMake command "FetchContent_Declare".

[TheAssassin]

Thank you!