If you have 11 odd numbers added together the result is always odd, but 100 is an even number.
So we need a trick which causes at least one sheep to be in two rooms at once.
If we put one small room containing 9 sheep inside another larger room, we can have an even number of sheep in the larger room. Let's suppose we have 10 sheep in the larger room outside the small room containing 9 sheep, then the larger room effectively contains 19 sheep. We've used up two rooms in doing this so we have 9 rooms left to accommodate 100-19=81 sheep. Each of the other 9 rooms can contain 9 sheep, so we have 9×9+19=100 sheep.
Of course, we could simply have one sheep in a small room inside a larger room in which there were no other sheep. The 99 remaining sheep could be divided into the remaining 9 rooms, each containing 11 sheep: 9×11+1=100.