ppc: Include string.h for memset

This fixes build failures on ppc machines with a compiler that
supports -Werror=implicit-function-declaration.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/ppc/vp3dsp_altivec.c

@@ -18,6 +18,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <string.h>
+
 #include "config.h"
 #include "libavutil/attributes.h"
 #include "libavutil/cpu.h"