Ignore *.img images in files folder

Some (qcow) images have .img file name extension (e.g. Cirros).
Ignore such files in files/ folder too (as we already do
with .qcow2 and .gz images).

Change-Id: Iac8593b65205e25fd3f94244a136c584d9af8eab

Author: pshchelo

.gitignore

@@ -12,6 +12,7 @@ doc/files
 doc/build
 files/*.gz
 files/*.qcow2
+files/*.img
 files/images
 files/pip-*
 files/get-pip.py*