add the missing #include <functional> header to fix the compilation errors

Author: coketaste

modules/module6/examples/04_reduction_cuda.cu

@@ -29,6 +29,7 @@
 #include <cassert>
 #include <iomanip>
 #include <float.h>
+#include <functional>
 
 namespace cg = cooperative_groups;
 

modules/module6/examples/05_prefix_sum_cuda.cu

@@ -30,6 +30,7 @@
 #include <cassert>
 #include <iomanip>
 #include <numeric>
+#include <functional>
 
 namespace cg = cooperative_groups;
 

modules/module7/examples/01_sorting_cuda.cu

@@ -30,6 +30,7 @@
 #include <iomanip>
 #include <thrust/sort.h>
 #include <thrust/device_vector.h>
+#include <functional>
 
 namespace cg = cooperative_groups;