[gtk-deprecated] Fix endless loop with grid stepping

commit 5a788e5ebe93b72a787319d1204f72ebb9aabd0a introduced a potential
endless loop with grid_step() and DiaRuler to "compensate for missing
5 and 6 in 3,4,..,7,8 major ticks". The rounding problem can be solved
more correctly with ROUND(), rather than potentially doing the same
step over and over again.