[REG: 5.13.0 beta -> RC] qdoc: Custom include paths not parsed correctly

Commit 50a172f8 broke the way QDoc parses include paths passed
from qdocconf files by always prepending them with '-I'. While
this ensured that Clang uses them as include paths, QDoc no
longer finds module headers local to the documentation project
as '-I/path/to/module' is never a valid file system path.

This increases the qdoc warning count for modules that use custom module headers (i.e. 'moduleheader' qdocconf variable.