[libc++] Add missing #include in <cwchar> tests

Thanks to Mikhail Maltsev for the patch.
Differential Revision: https://reviews.llvm.org/D63289

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363290 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/depr/depr.c.headers/wchar_h.pass.cpp

@@ -10,6 +10,7 @@
 
 #include <wchar.h>
 #include <stdarg.h>
+#include <stdio.h>
 #include <type_traits>
 
 #include "test_macros.h"

test/std/strings/c.strings/cwchar.pass.cpp

@@ -11,6 +11,7 @@
 #include <cwchar>
 #include <ctime>
 #include <cstdarg>
+#include <cstdio>
 #include <type_traits>
 
 #include "test_macros.h"