Fix FTBFS with GCC7

Signed-off-by: François Andriot <francois.andriot@...>
(cherry picked from commit 4767f333ca8a7997b4c4f9ec3f2f4fdb6e59d7ea)

diff --git a/korganizer/koeditorattachments.cpp b/korganizer/koeditorattachments.cpp
index 4560d34..48b9acd 100644
--- a/korganizer/koeditorattachments.cpp
+++ b/korganizer/koeditorattachments.cpp
@@ -79,7 +79,7 @@
       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();