[Clang][OpenMP] Fix stdio.h wrapper when glibc includes again (#77017)

Since D154036 landed (2a65d038 on July 7, 2023), I've been seeing
many (40-50) libomptarget test failures with errors like the following
on some of our test systems:

```
/auto/software/gcc/x86_64/gcc-11.1.0/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0/cstdio:99:11: error: no member named 'fpos_t' in the global namespace
   99 |   using ::fpos_t;
      |         ~~^
```

This patch fixes that and doesn't break our other test sytems. I've
looked through the glibc history and at LLVM's libc stdio.h to give me
confidence that this solution should work on other systems. Of course,
there might be use cases I've overlooked, so feedback is appreciated.