Bug 648735 - OpenBSD: Include sys/stat.h; r=cjones

otherwise fails with:
ipc/chromium/src/base/platform_file_posix.cc:49: error: 'S_IRUSR' was not declared in this scope
ipc/chromium/src/base/platform_file_posix.cc:49: error: 'S_IWUSR' was not declared in this scope

ipc/chromium/src/base/platform_file_posix.cc

@@ -4,6 +4,7 @@
 
 #include "base/platform_file.h"
 
+#include <sys/stat.h>
 #include <fcntl.h>
 #include <errno.h>
 #ifdef ANDROID