Fix a couple of warnings in unit tests.
diff --git a/test/array.cpp b/test/array.cpp
index 631a294..330749e 100644
--- a/test/array.cpp
+++ b/test/array.cpp
@@ -152,6 +152,7 @@
 template<typename ArrayType> void array_real(const ArrayType& m)
 {
   using std::abs;
+  using std::sqrt;
   typedef typename ArrayType::Index Index;
   typedef typename ArrayType::Scalar Scalar;
   typedef typename NumTraits<Scalar>::Real RealScalar;