Doc: Set an empty module header for QtDoc

Clang tries to find a module header <QtDoc> for this project and warns
about it. Use an empty 'moduleheader' variable to clear the warning.

Change-Id: Iee7ae73339f5e345c25d2e9ded0050378b476356
Reviewed-by: Martin Smith <martin.smith@qt.io>

doc/config/qtdoc.qdocconf

@@ -6,6 +6,9 @@ project                 = QtDoc
 description             = Qt Reference Documentation
 version                 = $QT_VERSION
 
+# Instruct Clang not to look for a module header
+moduleheader =
+
 depends += \
     activeqt \
     qdoc \