vorbis: vpxenc: Add missing include for av_rescale*

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/libvorbis.c

@@ -30,6 +30,7 @@
 #include "avcodec.h"
 #include "bytestream.h"
 #include "vorbis.h"
+#include "libavutil/mathematics.h"
 
 #undef NDEBUG
 #include <assert.h>

libavcodec/libvpxenc.c

@@ -30,6 +30,7 @@
 
 #include "avcodec.h"
 #include "libavutil/base64.h"
+#include "libavutil/mathematics.h"
 
 /**
  * Portion of struct vpx_codec_cx_pkt from vpx_encoder.h.