hidl-gen: enumerate interface methods in Bn classes

Bn${interface} classes inherit from I${interface} as as from IBinder
(via BBinder).  Make this explicit in auto-generated code by enumerating
the methods from I${interface} as pure virtual ones.  This is a no-op
change--it's just for clarity.

Change-Id: Idc01757c92613a9b0461bb57ed269348fab4a40a
Signed-off-by: Iliyan Malchev <malchev@google.com>
diff --git a/AST.h b/AST.h
index 97ecc6f..2d674ba 100644
--- a/AST.h
+++ b/AST.h
@@ -93,6 +93,9 @@
     status_t generateTypeSource(
             Formatter &out, const std::string &ifaceName) const;
 
+    status_t generateHeaderMethodSignatures(
+            Formatter &out, bool abstract) const;
+
     status_t generateProxySource(
             Formatter &out, const std::string &baseName) const;