Add patch file to fix guix-gaming-channels

This patch file should be discoverable through the load path. I'm committing
this here temporarily to see if updating my package repository helps to fix my
problems with ‘guix package -u’ (can't find patch file).

mono-mdoc-timestamping.patch

@@ -0,0 +1,15 @@
+--- mono-4.4.1/mcs/class/monodoc/Monodoc/storage/ZipStorage.cs.orig	2018-11-26 22:16:25.008879747 +0100
++++ mono-4.4.1/mcs/class/monodoc/Monodoc/storage/ZipStorage.cs	2018-11-26 22:21:53.969770985 +0100
+@@ -74,6 +74,12 @@
+ 				id = GetNewCode ();
+ 
+ 			ZipEntry entry = new ZipEntry (id);
++			var SOURCE_DATE_EPOCH_string = Environment.GetEnvironmentVariable("SOURCE_DATE_EPOCH");
++			if (SOURCE_DATE_EPOCH_string != null)
++			{
++				var SOURCE_DATE_EPOCH = Convert.ToInt64(SOURCE_DATE_EPOCH_string);
++				entry.DateTime = new DateTime(SOURCE_DATE_EPOCH, DateTimeKind.Utc);
++			}
+ 			zipOutput.PutNextEntry (entry);
+ 		}
+