build failure with ghc 8.4

Add an instance for semigroup.

```
Building library for timespan-0.3.0.0..
[1 of 1] Compiling Data.Time.TimeSpan ( src/Data/Time/TimeSpan.hs, dist/build/Data/Time/TimeSpan.o )

src/Data/Time/TimeSpan.hs:27:10: error:
• No instance for (Semigroup TimeSpan)
arising from the superclasses of an instance declaration
• In the instance declaration for ‘Monoid TimeSpan’
|
27 | instance Monoid TimeSpan where
| ^^^^^^^^^^^^^^^
```