arch: POSIX: Add guard into posix_cheats file.

Some of APIs of POSIX implmentation layer has same name as
native_posix architecture. posix_cheats.h is used to handle this
duplication in API name. Adding a guard in posix_cheats.h based on
CONFIG_PTHREAD_API.

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>