Add include(iree_add_all_subdirs) to main CMakeLists

Accidentally reverted this line while merging https://github.com/google/iree/pull/1002

Closes https://github.com/google/iree/pull/1010

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/1010 from GMNGeoffrey:bazel-to-cmake 4330264a37ac710a7aa046f73d0586bd4bd54568
PiperOrigin-RevId: 299897898
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 354b0de..ee913be 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -77,6 +77,7 @@
 include(iree_py_test)
 include(iree_lit_test)
 include(iree_alwayslink)
+include(iree_add_all_subdirs)
 
 string(JOIN " " CMAKE_CXX_FLAGS ${IREE_DEFAULT_COPTS})