Add a .gitignore to Assistant

These files are generated in the source dir, unfortunately.

Change-Id: Ib3d64ee817a5217a04a9a39005201df0ac23691c
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>

src/assistant/.gitignore

+clucene/qtcluceneversion.h
+help/qthelpversion.h