Fix r217275 to work without the need for standard headers being included

It seems (I guess) in ObjC that va_list is provided without the need for
inclusions. I verified that with this change the test still crashes in
the absence of the fix committed in r217275.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217290 91177308-0d34-0410-b5e6-96231b3b80d8