debian: add debhelper tokens to postinst/prerm scripts.

Fixes maintainer-script-lacks-debhelper-token lintian warning.

debian/nginx.postinst

diff --git a/debian/nginx.postinst b/debian/nginx.postinst
index e0b6e6b..8edc9e6 100644
--- a/debian/nginx.postinst
+++ b/debian/nginx.postinst
@@ -36,3 +36,7 @@
         fi
     fi
 fi
+
+#DEBHELPER#
+
+exit 0

debian/nginx.prerm

diff --git a/debian/nginx.prerm b/debian/nginx.prerm
index 7c26a4f..5f15bd8 100644
--- a/debian/nginx.prerm
+++ b/debian/nginx.prerm
@@ -19,4 +19,6 @@
         ;;
 esac
 
+#DEBHELPER#
+
 exit 0