algorithms_cpp: removed cppcheck style warnings

algorithms_cpp/test/count_test.cc

@@ -32,7 +32,7 @@
 #include <functional>
 
 struct IsEven{
-  bool operator()(int val) {
+  bool operator()(int val) const {
     if (val % 2 == 0)
       return true;
     else

algorithms_cpp/test/for_each_test.cc

@@ -38,7 +38,7 @@
  */
 struct Square {
   template<typename Type>
-  void operator()(Type& l) {
+  void operator()(Type& l) const {
     l = l * l;
   }
 };

algorithms_cpp/test/reduce_test.cc

@@ -36,7 +36,7 @@
  */
 struct Square {
   template<typename Type>
-  Type operator()(Type& l) {
+  Type operator()(Type& l) const {
     return l * l;
   }
 };

algorithms_cpp/test/scan_test.cc

@@ -35,7 +35,7 @@
  */
 struct Square {
   template<typename Type>
-  Type operator()(Type& l) {
+  Type operator()(Type& l) const {
     return l * l;
   }
 };