Convert calls to CHECK_NOTNULL to CHECK.

CHECK_NOTNULL is being deprecated and removed from the
glog library.

Change-Id: I4a6d1eec6e82a768c7861c8f776bf1f9c0b50c74
diff --git a/include/ceres/dynamic_cost_function_to_functor.h b/include/ceres/dynamic_cost_function_to_functor.h
index d4fce1c..7ae5291 100644
--- a/include/ceres/dynamic_cost_function_to_functor.h
+++ b/include/ceres/dynamic_cost_function_to_functor.h
@@ -105,7 +105,7 @@
   // Takes ownership of cost_function.
   explicit DynamicCostFunctionToFunctor(CostFunction* cost_function)
       : cost_function_(cost_function) {
-    CHECK_NOTNULL(cost_function);
+    CHECK(cost_function != nullptr);
   }
 
   bool operator()(double const* const* parameters, double* residuals) const {