Use https to load docbookx.dtd.

Fixes build failures on code's Win32 VM. It seemed not to be a problem anywhere else, but it
should be harmless.

Author: jralls

docbook/gnc-docbookx.dtd

@@ -18,7 +18,7 @@ own entity definitions to use across all of our DocBook pages. -->
 ***********************************************
 -->
 <!ENTITY % orig-docbook PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
-                               "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
+                               "https://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
 %orig-docbook;
 
 <!--