MLECO-3110: Fix for warnings from TensorFlow Lite Micro

Changes to fix "Pillow not found" warnings from TensorFlow
Lite Micro build.

Change-Id: I42a9b3f6df746ac3e34585e0c79ffdf35da97aef
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>
diff --git a/scripts/cmake/source_gen_utils.cmake b/scripts/cmake/source_gen_utils.cmake
index cd2698c..29efc93 100644
--- a/scripts/cmake/source_gen_utils.cmake
+++ b/scripts/cmake/source_gen_utils.cmake
@@ -277,6 +277,7 @@
         set(PYTHON ${DEFAULT_VENV_DIR}/bin/${PY_EXEC})
     endif()
     set(PYTHON ${PYTHON} PARENT_SCOPE)
+    set(PYTHON_VENV ${DEFAULT_VENV_DIR} PARENT_SCOPE)
 
     if (EXISTS ${PYTHON})
         message(STATUS "Using existing python at ${PYTHON}")