PblWML schema: add the "dependencies" attribute

There's also a "translate" attribute which is missing in the schema,
this commit deliberately omits it as it was for WesCamp.

The commit comment in e6d1595f8 is wrong - the validator does complain
about the error fixed in that commit, I'd missed the --use-schema option.

data/schema/pbl.cfg

@@ -22,6 +22,7 @@
 		{REQUIRED_KEY author string}
 		{REQUIRED_KEY passphrase string}
 		{REQUIRED_KEY description string}
+		{SIMPLE_KEY dependencies string}
 		{SIMPLE_KEY tags string}
 		{SIMPLE_KEY type addon_type}
 		{SIMPLE_KEY email email}