lintian: Add explicit source format

Change-Id: I09bb6b50378f12112ada991fd0b7c2097447655a
(cherry picked from commit 35232fce57220ddd8d513b3da517c48cc3d0a989)
diff --git a/debian/source/format b/debian/source/format
new file mode 100644
index 0000000..d3827e7
--- /dev/null
+++ b/debian/source/format
@@ -0,0 +1 @@
+1.0