[vala/staging] tests: Add missing PrintfFormat attribute

[Rico Tzschichholz <ricotz src gnome org>]

commit e86aa46f8b336f14d1e230a350d8ccf5453ac2f6
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Sat Jan 27 19:30:49 2018 +0100

    tests: Add missing PrintfFormat attribute
    
    https://bugzilla.gnome.org/show_bug.cgi?id=657378

 tests/structs/bug657378.vala |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/tests/structs/bug657378.vala b/tests/structs/bug657378.vala
index 5ba8263..91c2196 100644
--- a/tests/structs/bug657378.vala
+++ b/tests/structs/bug657378.vala
@@ -1,3 +1,4 @@
+[PrintfFormat]
 string foo (string format, ...) {
        return format.vprintf (va_list ());
 }