lintian: Add explicit source format

Change-Id: I8438346800b6d14927b29daaa860329c414b8420
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