[NFC] Fix `-Wcast-qual` warnings in Darwin `internal_sysctl(...)` implementation.

Summary: The warnings were introduced in r341187 and r341192.

Reviewers: kubamracek, george.karpenkov, krytarowski

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D52717

llvm-svn: 343496