Prevent avcodec.h from redefining SampleFormat.

Without this patch, the SampleFormat token will be redefined to
AVSampleFormat even if it occurs in some other scope. One example is
osg::AudioStream::SampleFormat, which the preprocessor turns into
osg::AudioStream::AVSampleFormat, causing compiler errors like this:
error: ‘AVSampleFormat’ in class ‘osg::AudioStream’ does not name a type

Looking at the code in ffmpeg, this patch should become unnecessary
once LIBAVCODEC_VERSION_MAJOR turns 53.

2011-02-04 Martin von Gagern

References:
http://bugs.gentoo.org/347481
http://git.ffmpeg.org/?p=ffmpeg.git;a=commitdiff;h=262d1c5d2206467809fd81d841d09ad70cc8ba13

Index: OpenSceneGraph-2.8.3/src/osgPlugins/ffmpeg/FFmpegHeaders.hpp
===================================================================
--- OpenSceneGraph-2.8.3.orig/src/osgPlugins/ffmpeg/FFmpegHeaders.hpp
+++ OpenSceneGraph-2.8.3/src/osgPlugins/ffmpeg/FFmpegHeaders.hpp
@@ -6,6 +6,7 @@
 extern "C"
 {
 #define __STDC_CONSTANT_MACROS
+#define FF_API_OLD_SAMPLE_FMT 0
 #include <errno.h>    // for error codes defined in avformat.h
 #include <stdint.h>
 #include <avcodec.h>