[mlir] mark libraries in mlir/examples as such

LLVM build system separates between `add_llvm_example_library` and
`add_llvm_library`, which is presumably used to package examples
separately from the regular library. Introduce a similar approach to
building example libraries in MLIR and use it for the transform dialect
tutorial.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D153265