Add missing mlir-headers target and add tablegen'd deps to it.

Summary:
Prior to this, "ninja install-mlir-headers" failed with an error indicating
the missing target. Verified that from a clean build, the installed
headers include generated files.

Subscribers: mgorny, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72045