avcodec/libspeexdec: fix missing header includes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libspeexdec.c

@@ -23,6 +23,7 @@
 #include <speex/speex_stereo.h>
 #include <speex/speex_callbacks.h>
 #include "avcodec.h"
+#include "internal.h"
 
 typedef struct {
     AVFrame frame;