Qbs: Add Qbs doc output to .gitignore

So that the HTML files don't clutter Git Gui
when committing changes.

Change-Id: If415e42d3dd76274aaadce839593e8b1a4651b3b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

.gitignore

@@ -110,6 +110,7 @@ phony.c
 /dist/gdb/qtcreator-*/
 /dist/gdb/source/
 /dist/gdb/staging/
+/doc/qbs/
 /doc/qtcreator/
 /doc/qtcreator-dev/
 /doc/pluginhowto/html/