add multiple inclusion guards to headers

Originally committed as revision 9345 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/xiph.h

@@ -18,6 +18,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#ifndef AVCODEC_XIPH_H
+#define AVCODEC_XIPH_H
+
 #include "common.h"
 
 /**
@@ -36,3 +39,5 @@
 int ff_split_xiph_headers(uint8_t *extradata, int extradata_size,
                           int first_header_size, uint8_t *header_start[3],
                           int header_len[3]);
+
+#endif