[SecuritySolution] Clean up loading state when the timeline import mo…

…dal closes (#197018)

## Summary

Fixes: #196877

The above issue describes an issue where the timeline import modal can
get into a broken state after a timeline failed to import. The fix was
to synchronize the loading state of the modal.

x-pack/plugins/security_solution/public/common/components/import_data_modal/index.tsx

@@ -91,6 +91,7 @@ export const ImportDataModalComponent = ({
     setOverwriteExceptions(false);
     setOverwriteActionConnectors(false);
     setActionConnectorsWarnings([]);
+    setIsImporting(false);
   }, [closeModal, setOverwrite, setOverwriteExceptions]);
 
   const onImportComplete = useCallback(