Include alsa headers before the internal FFmpeg headers.

Include alsa headers before the internal FFmpeg headers.

This avoids symbol redefinitions problems, for example avoids the "free"
symbol to be redefined before system headers actually using it are
included, thus breaking compilation. In particular this change allows
to build FFmpeg with salsa.

Patch by matthieu castet <$surname.mat?hieu@free fr>.

Originally committed as revision 20665 to svn://svn.ffmpeg.org/ffmpeg/trunk