Update .gitignore to exclude zephyr sub-git

Change-Id: I326fcf3d7c9bcdb4e4d8b5ea1e36660dfe4e5ae7
diff --git a/.gitignore b/.gitignore
index e385464..c90f033 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 /openamp
 /rtos/freertos
 /rtos/threadx
+/rtos/zephyr
 /tensorflow
 /tflite_micro
 /cmsis-view