Add missing copy-op to fix a cppcheck warning

This should trigger a build whilst fixing a single warning

Framework/Kernel/inc/MantidKernel/MultiFileValidator.h

@@ -29,9 +29,13 @@ namespace Kernel {
 class MANTID_KERNEL_DLL MultiFileValidator : public TypedValidator<std::vector<std::vector<std::string>>> {
 public:
   MultiFileValidator();
-  MultiFileValidator(const MultiFileValidator &mfv);
   explicit MultiFileValidator(const std::vector<std::string> &extensions, bool testFilesExist = true);
 
+  // Copy constructor / operator
+  friend void swap(MultiFileValidator &obj1, MultiFileValidator &obj2);
+  MultiFileValidator(const MultiFileValidator &mfv);
+  MultiFileValidator &operator=(MultiFileValidator);
+
   IValidator_sptr clone() const override;
 
   /// Returns the set of allowed extensions.

Framework/Kernel/src/MultiFileValidator.cpp

@@ -24,6 +24,16 @@ MultiFileValidator::MultiFileValidator()
 MultiFileValidator::MultiFileValidator(const MultiFileValidator &mfv)
     : TypedValidator<std::vector<std::vector<std::string>>>(), m_fileValidator(mfv.m_fileValidator) {}
 
+void swap(MultiFileValidator &obj1, MultiFileValidator &obj2) {
+  using std::swap;
+  swap(obj1.m_fileValidator, obj2.m_fileValidator);
+}
+
+MultiFileValidator &MultiFileValidator::operator=(MultiFileValidator tmp) {
+  swap(*this, tmp);
+  return *this;
+}
+
 /** Constructor
  *  @param extensions :: The permitted file extensions (e.g. .RAW)
  *  @param testFilesExist :: If to check if files exist