CLEANUP: don't ignore debian/ directory if present

This makes packaging with git tools easier as it is not possible to
cancel anything in debian/.gitignore since the whole directory is
ignored.
(cherry picked from commit 638f66ac05f092400676ccd28f44a2869a88d5d8)
diff --git a/.gitignore b/.gitignore
index 1953ba3..a7b557a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -65,3 +65,4 @@
 !/include
 !/src
 !/tests
+!/debian