[libcxx] [test] D27013: Fix MSVC warning C4018 "signed/unsigned mismatch", part 1/12.

Change loop indices from int to std::size_t.

Also, include <cstddef> when it wasn't already being included.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287820 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/container.adaptors/stack/stack.cons.alloc/ctor_container_alloc.pass.cpp

@@ -14,6 +14,7 @@
 
 #include <stack>
 #include <cassert>
+#include <cstddef>
 
 #include "test_allocator.h"
 
@@ -49,7 +50,7 @@ int main()
     test q(d, test_allocator<int>(4));
     assert(q.get_allocator() == test_allocator<int>(4));
     assert(q.size() == 5);
-    for (int i = 0; i < d.size(); ++i)
+    for (std::size_t i = 0; i < d.size(); ++i)
     {
         assert(q.top() == d[d.size() - i - 1]);
         q.pop();