Linker command failed

Compiled using Swift version - **4.1.2**
HandyJSON Library version - **4.2.0** .
I am getting some linking error

```
Undefined symbols for architecture x86_64:
"_swift_getFieldAt", referenced from:
HandyJSON.Metadata.Class._propertyDescriptionsAndStartPoint() -> ([HandyJSON.Property.Description], Swift.Int32?)? in Metadata.o
HandyJSON.Metadata.Struct.propertyDescriptions() -> [HandyJSON.Property.Description]? in Metadata.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

```

It works fine with library version - **4.1.0**
It will be really helpful if you can give solution to above problem. Thank you :)

Start by reproducing the linker failure with Swift 4.1.2 and HandyJSON 4.2.0, then compare the build with library version 4.1.0. Done means identifying a compatible configuration or change that resolves the undefined _swift_getFieldAt symbol for x86_64.

Written by the indexing model from the issue text.