[2218] define the default ctor for TestNSEC3HashCreator.

clang requires it to construct a const object of it.

src/lib/datasrc/tests/faked_nsec3.h

@@ -57,6 +57,7 @@ class TestNSEC3HashCreator : public isc::dns::NSEC3HashCreator {
 private:
     class TestNSEC3Hash;
 public:
+    TestNSEC3HashCreator() {}
     virtual isc::dns::NSEC3Hash* create(const
                                         isc::dns::rdata::generic::NSEC3PARAM&)
         const;