[mlir] Fix Windows build

Summary:
MSVC does not seem to like certain forward declarations.

https://reviews.llvm.org/D80728 introduces an error where
seemingly unrelated .cpp files that include the .h
(but do not otherwise use the class that depends on the forward declaration).

Instead of forward declaration, include the full vector ops definition.

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80841