[Sema] attempt to appease buildbots after r367940

A buildbot got angry about this new test, with error messages like:

warn-nullchar-nullptr.c Line 16: use of undeclared identifier 'u'

It looks like this `u'c'` syntax was introduced in C11; I'm guessing
some bots may default to something before that. Let's see if explicitly
specifying the standard version makes it happy...

llvm-svn: 367947

clang/test/Sema/warn-nullchar-nullptr.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -Wall %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wall %s -std=c11
 
 int test1(int *a) {
   return a == '\0'; // expected-warning {{comparing a pointer to a null character constant; did you mean to compare to (void *)0?}}