added `pthread_cond.h` to `pthread.h`
adjusted `test_pthread_condition_variable\main.c` includes reflecting the above change
Merge pull request #1117 from BytesGalore/add_missing_pthread_cv_header

sys:posix:pthread added missing `pthread_cond.h` to `pthread.h`
core/cpu: put __builtin_unreachable() into one place