NVOS_IS_UNIX is not defined

When including nvapi.h in a project that has MSVC warning C4668 enabled, the following is emitted

nvapi\nvapi_lite_common.h(73,7): warning C4668: 'NVOS_IS_UNIX' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'

While this is minor, it would make for a cleaner compile if this macro could be defined (or if the feature test could checks for the macro being defined ahead of trying to evaluate it)