[scudo] Do not include sanitizer_posix.h if not on a Posix platform

Summary:
Move the `sanitizer_posix.h` include within the `SANITIZER_ANDROID` `#if`,
otherwise this errors when built on non-Posix platforms (eg: Fuchsia).

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38956

llvm-svn: 315917