Fix test failure on Linux.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55032 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/Sema/format-strings.c b/test/Sema/format-strings.c
index 2d6e719..ec070a2 100644
--- a/test/Sema/format-strings.c
+++ b/test/Sema/format-strings.c
@@ -1,5 +1,8 @@
 // RUN: clang -fsyntax-only -verify %s
 
+// Define this to get vasprintf on Linux
+#define _GNU_SOURCE
+
 #include <stdio.h>
 #include <stdarg.h>