[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

GitOrigin-RevId: 14d9ef29304ee16802657311911813aec5bdb9cf
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ac98cc2..bd3d309 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -260,3 +260,7 @@
 if (MLIR_ENABLE_PYTHON_BENCHMARKS)
   add_subdirectory(utils/mbr)
 endif()
+
+if(MLIR_STANDALONE_BUILD)
+  llvm_distribution_add_targets()
+endif()