Fix warning conflict on `solve!`

[tmigot]

┌ DCISolver [bee2e536-65f6-11e9-3844-e5bb4c9c55c9]
│  WARNING: using Krylov.solve! in module DCISolver conflicts with an existing identifier.
└  

[amontoison]

I will unexport Krylov.solve! with the next major release.