Fix access mode for ::popen() call in ggp OS wrapper
Fix access mode for ::popen() call in ggp OS wrapper

popen's access mode was accidentally changed from string to enum used in
FileIO::open() wrappers. This is a posix function and should keep using
the string.

baldurk/renderdoc,baldurk/renderdoc,baldurk/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,baldurk/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,baldurk/renderdoc,baldurk/renderdoc