fixing a compilation error

Author: Velin92

MatrixSDK/Background/MXBackgroundStore.swift

@@ -298,6 +298,11 @@ class MXBackgroundStore: NSObject, MXStore {
     func isRoomMarked(asUnread roomId: String) -> Bool {
         return false
     }
+    
+    func removeAllMessagesSent(before limitTs: UInt64, inRoom roomId: String) -> Bool {
+        // Not sure if this needs to be implemented
+        false
+    }
 }
 
 //  MARK: - MXRoomSummaryStore
@@ -334,5 +339,4 @@ extension MXBackgroundStore: MXRoomSummaryStore {
             completion([])
         }
     }
-    
 }