lintian: Add explicit source format

Change-Id: I707edf72bb5553d55e92928bdf442651966c1ed9
(cherry picked from commit ad067873ca2343c6307ed50f2070be5e57944699)
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