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.
diff --git a/.gitignore b/.gitignore
index 45ff1c3..0292bcc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -68,3 +68,4 @@
 !/include
 !/src
 !/tests
+!/debian