Removed a direct build dependency on dh-systemd

The package is obsoleted and removed from contemporary Ubuntu and Debian
versions.  It's advised by lintian to use a specific debhelper version instead.

debian/nginx.control.in

diff --git a/debian/nginx.control.in b/debian/nginx.control.in
index c44413d..439dd7f 100644
--- a/debian/nginx.control.in
+++ b/debian/nginx.control.in
@@ -3,7 +3,7 @@
 Priority: optional
 Maintainer: %%PACKAGE_VENDOR%%
 Build-Depends: debhelper (>= 9~),
-               dh-systemd,
+               dh-systemd | debhelper (>= 9.20160709),
                dpkg-dev (>= 1.16.1~),
                quilt (>= 0.46-7~),
                lsb-release,