add secondary_authors to pbl schema

data/schema/pbl.cfg

@@ -1,6 +1,7 @@
 [wml_schema]
 	{./macros.cfg}
 	{./types/basic.cfg}
+	{./types/lists.cfg}
 	[type]
 		name="version"
 		value="\d+\.\d+.*"
@@ -41,6 +42,9 @@
 				{SIMPLE_KEY passphrase string}
 				{SIMPLE_KEY email email}
 			[/then]
+			[else]
+				{SIMPLE_KEY secondary_authors string_list}
+			[/else]
 		[/if]
 		[tag]
 			name="feedback"