Merge pull request #123 from anishmonachan7/fix_unused_variable

fix unused variable error with gcc 8.3

include/valijson/constraints/concrete_constraints.hpp

@@ -914,6 +914,9 @@ public:
             freeFn(ptr);
             throw;
         }
+#else
+        // pretend to use freeFn to avoid warning in GCC 8.3
+        (void)freeFn;
 #endif
     }