Build: Make the app build if JitsiMeetSDK is not in the Podfile.

We already had a mechanism to automatically build VoIP according to the presence of the webrtc pod file. Do the same but for conference calls.

Riot/Modules/Integrations/Widgets/Jitsi/JitsiService.swift

@@ -15,6 +15,8 @@
  */
 
 import Foundation
+
+#if canImport(JitsiMeet)
 import JitsiMeet
 
 /// JitsiService enables to abstract and configure Jitsi Meet SDK
@@ -72,3 +74,4 @@ final class JitsiService: NSObject {
         return self.jitsiMeet.application(application, continue: userActivity, restorationHandler: restorationHandler)
     }
 }
+#endif

Riot/Modules/Integrations/Widgets/Jitsi/JitsiViewController.m

@@ -17,6 +17,7 @@
 #import "JitsiViewController.h"
 #import "JitsiWidgetData.h"
 
+#if __has_include(<MatrixSDK/MXJingleCallStack.h>)
 @import JitsiMeet;
 
 static const NSString *kJitsiDataErrorKey = @"error";
@@ -251,3 +252,5 @@ static const NSString *kJitsiDataErrorKey = @"error";
 }
 
 @end
+
+#endif