debian: install the man page for nginx.

Fixes binary-without-manpage lintian warning.
diff --git a/debian/nginx.manpages b/debian/nginx.manpages
new file mode 100644
index 0000000..7497da2
--- /dev/null
+++ b/debian/nginx.manpages
@@ -0,0 +1 @@
+objs/nginx.8