Added pragma once guards to header files

Operation.h

@@ -1,3 +1,5 @@
+#pragma once
+
 enum class OpType
 {
     Addition, Subtraction, Multiplication, Division, Modulo, Special

basecalc.h

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "Operation.h"
 class BaseCalculator
 {

command.h

@@ -1,3 +1,5 @@
+#pragma once
+
 class Command
 {
     public:

input_resolver.h

@@ -1,2 +1,4 @@
+#pragma once
+
 #include "command.h"
 Command resolveCommand();

special_calculator.h

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "basecalc.h"
 
 class SpecialCalculator: public BaseCalculator