Keep cstdbool Header Behind cpp Macros

[alifarazz]

When compiling NVSHMEM using GCC 15.2.1 and CUDA 13.0 (nvcc release 13.0, V13.0.88), I get the following warning:

...
/usr/include/c++/15.2.1/cstdbool:50:6: warning: #warning "<cstdbool> is deprecated in C++17, remove the #include" [-Wcpp]
   50 | #    warning "<cstdbool> is deprecated in C++17, remove the #include"
      |      ^~~~~
...

This PR fixes the warning by putting cstdbool behind a preprocessor macro, in files that include it. cstdbool was deprecated in C++17 and removed in C++20 ¹.

Footnotes

1. https://en.cppreference.com/w/cpp/header/cstdbool.html ↩