fix compiler error for template pattern with swift 5.2

Author: David Orriss Jr

Design-Patterns.playground/Pages/Behavioral.xcplaygroundpage/Contents.swift

@@ -636,9 +636,9 @@ protocol Garden {
 extension Garden {
 
  func prepareGarden() {
- func prepareSoil()
- func plantSeeds()
- func waterPlants()
+ func prepareSoil() {}
+ func plantSeeds() {}
+ func waterPlants() {}
  }
 }