vp8cx_set_ref: fix variable assignment

assign vpx_ref_frame_t members explicitly avoiding incompatibility in
visual studio builds

Change-Id: I22df4c5ebb772d65b4d72f22949fbde53543d7ec

examples/vp8cx_set_ref.c

diff --git a/examples/vp8cx_set_ref.c b/examples/vp8cx_set_ref.c
index 39ac831..9b6d11b 100644
--- a/examples/vp8cx_set_ref.c
+++ b/examples/vp8cx_set_ref.c
@@ -162,7 +162,9 @@
 
   while (vpx_img_read(&raw, infile)) {
     if (frame_count + 1 == update_frame_num) {
-      vpx_ref_frame_t ref = {VP8_LAST_FRAME, raw};
+      vpx_ref_frame_t ref;
+      ref.frame_type = VP8_LAST_FRAME;
+      ref.img = raw;
       if (vpx_codec_control(&codec, VP8_SET_REFERENCE, &ref))
         die_codec(&codec, "Failed to set reference frame");
     }