QtAutogen: Only add source files to the target if AUTORCC is ON.

The qtx_add_resources() macro adds the resource file to the output list
to maintain file-level dependencies.  Having the qrc file in a target
sources is a precondition for AUTORCC to function.

When processing the source files of a target, only add the generated
qrc_<file>.cpp to the target sources if AUTORCC is ON.  This avoids
pre-porting conflict with the macro.

Reported-by: Micha Hergarden
Merge topic 'fix-Qt-autogen' into next

8f8edeb0 QtAutogen: Only add source files to the target if AUTORCC is ON.