Add fn signature query

crates/ra_hir/src/mock.rs

@@ -210,6 +210,7 @@ salsa::database_storage! {
             fn impls_in_module() for db::ImplsInModuleQuery;
             fn body_hir() for db::BodyHirQuery;
             fn body_syntax_mapping() for db::BodySyntaxMappingQuery;
+            fn fn_signature() for db::FnSignatureQuery;
         }
     }
 }