Add .gitignore to ignore all .hidl-autogen files

Don't track the temporary, auto-generated .hidl-autogen
files that are used for tracking HAL changes.

Change-Id: I08f5259091f3b4dc1f8d751f1349b28d875d0a71
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..61fedf2
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.hidl-autogen