Solo: add a missing params constraint for X puzzles.

Michael Quevillon points out that neither 2x1 nor 3x1 Solo can be made
into an X Sudoku puzzle, on the grounds that whatever number goes in
one corner of the grid is ruled out from both ends (and the centre, if
any) of the opposing diagonal, and hence the X constraint can't be
satisfied.

(Also fixed a spurious full stop on a neighbouring line.)