Doc: Set empty module header for the documentation project

This prevents a QDoc Clang C++ parser warning about a missing header.

Change-Id: Id14b17de72488a12108b583ed0673d0451ff5e15
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/imageformats/doc/qtimageformats.qdocconf

@@ -5,6 +5,9 @@ project     = QtImageFormats
 description = Qt Image Formats Documentation
 version     = $QT_VERSION
 
+# Instruct Clang to not look for a module header
+moduleheader =
+
 # Directories in which to search for files to document.
 # Paths are relative to the location of this file.
 # exampledirs += ../examples