vp56: Don't use DECLARE_ALIGN on a typedef name

Instead, use it on the first member, since by definition, if
any member is aligned, the whole struct must be, in order to
maintain that alignment.

Fixes compilation with some finicky compilers.

Idea for fix from Måns Rullgård.
Signed-off-by: NDerek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/vp56.h

@@ -36,9 +36,9 @@
 typedef struct vp56_context VP56Context;
 
 typedef struct {
-    int16_t x;
+    DECLARE_ALIGNED(4, int16_t, x);
     int16_t y;
-} DECLARE_ALIGNED(4, , VP56mv);
+} VP56mv;
 
 typedef void (*VP56ParseVectorAdjustment)(VP56Context *s,
                                           VP56mv *vect);