lintian: Add explicit source format

Change-Id: I90a7fa413cca2e1b9b2fa5a15dbe2eeab7f062d6
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