[cpprestsdk] Fix usage issue when building static

[JackBoosY]

According to the header file, the macro _NO_ASYNCRTIMP should be defined on static build:

#ifdef _NO_ASYNCRTIMP
#define _ASYNCRTIMP
#else // ^^^ _NO_ASYNCRTIMP ^^^ // vvv !_NO_ASYNCRTIMP vvv
#ifdef _ASYNCRT_EXPORT
#define _ASYNCRTIMP __declspec(dllexport)
#else // ^^^ _ASYNCRT_EXPORT ^^^ // vvv !_ASYNCRT_EXPORT vvv
#define _ASYNCRTIMP __declspec(dllimport)
#endif // _ASYNCRT_EXPORT
#endif // _NO_ASYNCRTIMP

Fixes #996.