[mlir][sparse] minor cleanup of include placement

Rationale:
empty line between main include for this file
moved include that actually defines code into right section

Note that this revision started as breaking up ops/attrs even more
(for bug https://github.com/llvm/llvm-project/issues/52748), but due
the the connection in Dialect.initalize(), this cannot be split further).
All heavy lifting refactoring was already done by River in previous cleanup.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D119617