qdoc: Add moduleheader as config variable

moduleheader is a new config variable you can add to the
qdocconf file when the name of the module header file for
your module is different from the project name. When the
name of the module header file is different from the
project name, if you don't tell qdoc what the name of the
module header file is by setting moduleheader in the
qdocconf file, then clangqdoc will not find the module
header file to build the precomiled header with, which
will result in clang not finding a lot of stuff.

Change-Id: I0da1a0b0be05cb9e6e95e0123583ddeedaf6741d
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>