chroma: copy: do not use an empty structure if SSE2 is not supported

Structures with no field should not be used, it's Undefined Behavior:
https://stackoverflow.com/questions/3849334/sizeof-empty-structure-is-0-in-c-and-1-in-c-why

modules/video_chroma/copy.h

@@ -29,6 +29,8 @@ typedef struct {
 # ifdef CAN_COMPILE_SSE2
     uint8_t *buffer;
     size_t  size;
+# else
+    char dummy;
 # endif
 } copy_cache_t;