Clang: Use timer in ClangIpcServer in the file watcher to send diagnostics

One mechanism for all diagnostic updates is better to maintain.

Change-Id: I774577bfa343162ded362977a0727a12379d1eab
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/tools/clangbackend/ipcsource/clangipcserver.h

@@ -42,6 +42,7 @@
 #include <utf8string.h>
 
 #include <QMap>
+#include <QTimer>
 
 namespace ClangBackEnd {