Fix missing include issue in exact_error_estimator.h.

When included alone, you apparently can't get away with just forward
declarations of TypeVector and TypeTensor. The error messages I was
getting were along the lines of:

In file included from /Users/petejw/projects/libmesh_git/installed/include/libmesh/exact_error_estimator.h:25:
/Users/petejw/projects/libmesh_git/installed/include/libmesh/function_base.h:136:18: error: implicit instantiation of undefined template 'libMesh::VectorValue<double>'
  virtual Output component(unsigned int i,
                 ^
/Users/petejw/projects/libmesh_git/installed/include/libmesh/function_base.h:77:11: note: in instantiation of member function 'libMesh::FunctionBase<libMesh::VectorValue<double> >::component' requested here
  virtual ~FunctionBase ();
          ^
/opt/moose/llvm-3.9.0/bin/../include/c++/v1/memory:2541:13: note: in instantiation of member function 'libMesh::FunctionBase<libMesh::VectorValue<double> >::~FunctionBase' requested here
            delete __ptr;
            ^
Refs libMesh#1593.

include/error_estimation/exact_error_estimator.h

@@ -23,6 +23,8 @@
 // Local Includes
 #include "libmesh/error_estimator.h"
 #include "libmesh/function_base.h"
+#include "libmesh/vector_value.h"
+#include "libmesh/tensor_value.h"
 
 // C++ includes
 #include <cstddef>
@@ -39,20 +41,8 @@ typedef FEGenericBase<Real> FEBase;
 class MeshFunction;
 class Point;
 class Parameters;
-
 template <typename T> class DenseVector;
 
-// Is there any way to simplify this?
-// All we need are Tensor and Gradient. - RHS
-template <typename T> class TensorValue;
-template <typename T> class VectorValue;
-typedef TensorValue<Number> NumberTensorValue;
-typedef NumberTensorValue   Tensor;
-typedef VectorValue<Number> NumberVectorValue;
-typedef NumberVectorValue   Gradient;
-
-
-
 /**
  * This class implements an "error estimator"
  * based on the difference between the approximate