Fix KSerialiser

Author: michaelprichardson

firebase-database/src/commonMain/kotlin/dev/gitlive/firebase/database/database.kt

@@ -9,6 +9,7 @@ import dev.gitlive.firebase.FirebaseApp
 import dev.gitlive.firebase.database.ChildEvent.Type.*
 import kotlinx.coroutines.flow.Flow
 import kotlinx.serialization.DeserializationStrategy
+import kotlinx.serialization.KSerializer
 import kotlinx.serialization.SerializationStrategy
 
 /** Returns the [FirebaseDatabase] instance of the default [FirebaseApp]. */