[MLIR] Enable distribution target in standalone builds

Invoke llvm_distribution_add_targets() when doing standalone build
explicitly in order to create the `distribution` target.

Differential Revision: https://reviews.llvm.org/D136269

Author: mgorny

mlir/CMakeLists.txt

@@ -260,3 +260,7 @@ add_subdirectory(cmake/modules)
 if (MLIR_ENABLE_PYTHON_BENCHMARKS)
   add_subdirectory(utils/mbr)
 endif()
+
+if(MLIR_STANDALONE_BUILD)
+  llvm_distribution_add_targets()
+endif()