More cleanup of win32.h.
PATH_MAX is defined if it is not already defined. A duplicate
definition is removed. The declaration of struct timespec is moved
outside #ifdef _MSC_VER to make it available for mingw.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@204449 91177308-0d34-0410-b5e6-96231b3b80d8