According Newton's 2nd law, the acceleration of the block is caused by the sum of all external forces acting upon it.
If the block is moving at constant speed (so it has no acceleration along the incline), this means that the sum of all external forces is 0.
Therefore, the work done on the block by all forces as the block moves a distance L, is just zero.
This can be expressed also saying that as the total work done on an object by a external net force, is equal to the change in kinetic energy, if the object is moving at a constant speed, this means that there is no change in the kinetic energy, so Wtotal must be 0.
