Fix FTBFS with GCC7

Signed-off-by: François Andriot <francois.andriot@free.fr>
(cherry picked from commit 4767f333ca)

korganizer/koeditorattachments.cpp

@@ -79,7 +79,7 @@ class AttachmentListItem : public TDEIconViewItem
       if ( att ) {
         mAttachment = new KCal::Attachment( *att );
       } else {
-        mAttachment = new KCal::Attachment( '\0' ); //use the non-uri constructor
+        mAttachment = new KCal::Attachment( TQChar('\0') ); //use the non-uri constructor
                                                     //as we want inline by default
       }
       readAttachment();