message: 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.
diff: @@ -181,7 +181,7 @@ private:
  uint64_t relative = addr - m_Modules[i].base + m_Modules[i].offset;
  rdcstr cmd = StringFormat::Fmt("addr2line -fCe \"%s\" 0x%llx", m_Modules[i].path, relative);
  - FILE *f = ::popen(cmd.c_str(), FileIO::ReadText);
  + FILE *f = ::popen(cmd.c_str(), "r");
  char result[2048] = {0};
  fread(result, 1, 2047, f);
  