Change test to just define NULL instead of #including stddef.h. In some

test environments, even that builtin header isn't available. Also, this
makes it more consistent with the C++ version of this test.

llvm-svn: 251520

clang-tools-extra/test/clang-tidy/modernize-redundant-void-arg.c

@@ -1,6 +1,6 @@
 // RUN: clang-tidy -checks=-*,modernize-redundant-void-arg %s -- -x c | count 0
 
-#include <stddef.h>
+#define NULL 0
 
 extern int i;