Fix bug in FF132 that causes MacOS builds to fail

Without --enable-backgroundtasks, macOS fails seem to fail after this change:
https://phabricator.services.mozilla.com/D219834

This commit patches the broken code in nsAppRunner.

patches/backgroudtasks-bugfix.patch

@@ -0,0 +1,21 @@
+diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
+index 318ee7e293..8430bb4e82 100644
+--- a/toolkit/xre/nsAppRunner.cpp
++++ b/toolkit/xre/nsAppRunner.cpp
+@@ -5640,10 +5640,14 @@ nsresult XREMain::XRE_mainRun() {
+
+     if (!AppShutdown::IsInOrBeyond(ShutdownPhase::AppShutdownConfirmed)) {
+ #ifdef XP_MACOSX
+-      if (!BackgroundTasks::IsBackgroundTaskMode()) {
++#ifdef MOZ_BACKGROUNDTASKS
++  if (!BackgroundTasks::IsBackgroundTaskMode()) {
++#endif
+         rv = appStartup->CreateHiddenWindow();
+         NS_ENSURE_SUCCESS(rv, NS_ERROR_FAILURE);
+-      }
++#ifdef MOZ_BACKGROUNDTASKS
++  }
++#endif
+ #endif
+
+ #ifdef XP_WIN