[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/numerics/numarray/valarray.nonmembers/valarray.transcend/pow_valarray_valarray.pass.cpp

@@ -18,6 +18,7 @@
 #include <valarray>
 #include <cassert>
 #include <sstream>
+#include <cstddef>
 
 bool is_about(double x, double y, int p)
 {
@@ -47,7 +48,7 @@ int main()
         std::valarray<T> v2(a2, N);
         std::valarray<T> v3 = pow(v1, v2);
         assert(v3.size() == v1.size());
-        for (int i = 0; i < v3.size(); ++i)
+        for (std::size_t i = 0; i < v3.size(); ++i)
             assert(is_about(v3[i], a3[i], 10));
     }
 }