lavfi/asyncts: Fix compilation with GCC 4.7

Compilation failed since commit 42d621d1
Error messages:
libavfilter/af_asyncts.c:249:5: error: initializer element is not
computable at load time
libavfilter/af_asyncts.c:249:5: error: (near initialization for
‘avfilter_af_asyncts.priv_size’)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_asyncts.c

@@ -246,5 +246,5 @@ AVFilter avfilter_af_asyncts = {
                                             .config_props   = config_props,
                                             .request_frame  = request_frame },
                                           { NULL }},
-    .priv_size = &asyncts_class,
+    .priv_class = &asyncts_class,
 };