Add attribute_align_arg to os2thread.c.

Patch by Dave Yeo.

Originally committed as revision 12447 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/os2thread.c

@@ -39,7 +39,7 @@ typedef struct ThreadContext{
 }ThreadContext;
 
 
-void thread_func(void *v){
+void attribute_align_arg thread_func(void *v){
     ThreadContext *c= v;
 
     for(;;){