Add missing #include <string.h> in x86/cpu.c

Originally committed as revision 25088 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/x86/cpu.c

@@ -21,6 +21,7 @@
  */
 
 #include <stdlib.h>
+#include <string.h>
 #include "libavutil/x86_cpu.h"
 #include "libavutil/cpu.h"