Meta: export "includes" dfn. (#530)

Closes #529.

Author: tobie

index.bs

@@ -1299,7 +1299,8 @@ used to declare that all objects implementing an [=interface=] |I|
 (identified by the first [=identifier=])
 must additionally include the [=interface mixin member|members] of [=interface mixin=] |M|
 (identified by the second identifier).
-[=Interface=] |I| is said to <dfn>include</dfn> [=interface mixin=] |M|.
+[=Interface=] |I| is said to <dfn id="include" for="interface" export>include</dfn>
+[=interface mixin=] |M|.
 
 <pre highlight="webidl" class="syntax">
     interface_identifier includes mixin_indentifier;