.gitignore: Ignore generated "common.env"

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/.gitignore b/.gitignore
index e32a584..090f974 100644
--- a/.gitignore
+++ b/.gitignore
@@ -109,3 +109,4 @@
 tags
 TAGS
 *~
+/tests/qemu-iotests/common.env