avcodec/nvdec: Make vp8 initialisation more 'compatible'

Ancient versions of gcc (pre 4.6) can't directly initialise
members of anonymous inner unions/structs by name.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10676

Unfortunately, RHEL 6 shipped with one of these ancient
versions and so we're stuck with it until approximately the
heat death of the universe.

Putting explicit braces into the initialisation is possibly a
work-around but the behaviour there was never fully understood
before direct initialisation was made to work.

So, this may or may not work.

libavcodec/nvdec_vp8.c

@@ -64,11 +64,20 @@ static int nvdec_vp8_start_frame(AVCodecContext *avctx, const uint8_t *buffer, u
             .LastRefIdx                  = safe_get_ref_idx(h->framep[VP56_FRAME_PREVIOUS]),
             .GoldenRefIdx                = safe_get_ref_idx(h->framep[VP56_FRAME_GOLDEN]),
             .AltRefIdx                   = safe_get_ref_idx(h->framep[VP56_FRAME_GOLDEN2]),
-
+            /*
+             * Explicit braces for anonymous inners to work around limitations
+             * in ancient versions of gcc.
+             */
+            {
+                {
                     .frame_type                  = !h->keyframe,
                     .version                     = h->profile,
                     .show_frame                  = !h->invisible,
-            .update_mb_segmentation_data = h->segmentation.enabled ? h->segmentation.update_feature_data : 0,
+                    .update_mb_segmentation_data = h->segmentation.enabled ?
+                                                   h->segmentation.update_feature_data :
+                                                   0,
+                }
+            }
        }
     };