Add necessary header for CONFIG_RUNTIME_CPUDETECT preprocessor definition.

Originally committed as revision 18774 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ppc/check_altivec.c

@@ -22,6 +22,8 @@
  * Checks for AltiVec presence.
  */
 
+#include "config.h"
+
 #ifdef __APPLE__
 #undef _POSIX_C_SOURCE
 #include <sys/sysctl.h>