[libc++] [test] Add missing required headers to midpoint.integer.pass.cpp

This change authored by Paolo Torres <t-pator@microsoft.com>

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358698 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.integer.pass.cpp b/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.integer.pass.cpp
index 32d931b..1956c97 100644
--- a/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.integer.pass.cpp
+++ b/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.integer.pass.cpp
@@ -13,6 +13,8 @@
 // _Tp midpoint(_Tp __a, _Tp __b) noexcept
 //
 
+#include <stdint.h>
+#include <limits>
 #include <numeric>
 #include <cassert>
 #include "test_macros.h"