Remove Plugins/Process/POSIX from include_directories

Summary:
The files there can always be referred to using their full path, which
is what most of the code has been doing already, so this makes the
situation more consistent. Also fix the the code in the FreeBSD plugin
to use the new paths.

Reviewers: eugene, emaste

Subscribers: lldb-commits, kettenis, mgorny, krytarowski

Differential Revision: https://reviews.llvm.org/D31877

llvm-svn: 299933