[MLIR] Fix build with make

https://reviews.llvm.org/D124075 causes MLIR to no longer build
when using make rather than ninja, due to a tablegen-generated
header being used before it is created.

It seems that this is related to the use of LLVM_ENABLE_OBJLIB when
using add_tablgen with a non-Ninja/Xcode generator. In that case an
intermediate objlib target is generated.

This patch fixes the issue by a) declaring dependencies in
add_tablegen for mlir-pdll and b) making sure those dependencies
are added to the objlib target.

Differential Revision: https://reviews.llvm.org/D125010