[libc][math] Fixed Hypotbf16 build failure. (#186415)
Ref from the build failure in hypotbf16
```CPP
project/libc/src/__support/math/hypotbf16.h:22:33: required from here
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian-fullbuild/libc-x86_64-debian-gcc-fullbuild-dbg/llvm-project/libc/src/__support/FPUtil/Hypot.h:221:44: error: conversion from ‘int’ to ‘StorageType’ {aka ‘short unsigned int’} may change value [-Werror=conversion]
221 | r = static_cast<StorageType>((r << 1)) +
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
222 | ((tail_bits & current_bit) ? 1 : 0);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
```
This PR intends to fix that by adding static_cast
