ppc: hpeldsp: Include attributes.h

This fixes building in configurations where altivec is disabled.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/ppc/hpeldsp_altivec.c

@@ -21,6 +21,7 @@
  */
 
 #include "config.h"
+#include "libavutil/attributes.h"
 #include "libavutil/cpu.h"
 #include "libavcodec/hpeldsp.h"