[libc][NFC] change isblank and iscntrl from implicit casting

isblank and iscntrl were casting an int to a char implicitly and this
was throwing errors under Fuchsia. I've added a static cast to resolve
this issue.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D94634

GitOrigin-RevId: ea8034ec35a9e3d6784d7e6f50617af3d87f6a9f
2 files changed