vpxdec: correct VP[89] fourccs

should have no effect as they are used in nestegg mappings, but aligns
the defines with vpxenc.c

Change-Id: Ic2295cd63701894c2963274239602b54cbb58631

vpxdec.c

@@ -49,8 +49,8 @@
 
 static const char *exec_name;
 
-#define VP8_FOURCC (0x00385056)
-#define VP9_FOURCC (0x00395056)
+#define VP8_FOURCC (0x30385056)
+#define VP9_FOURCC (0x30395056)
 static const struct {
   char const *name;
   const vpx_codec_iface_t *(*iface)(void);