Add some #includes to allow compilation without HAVE_AV_CONFIG_H.

taken from a patch by Ronald S. Bultje, rbultje ronald.bitfreak net

Originally committed as revision 9263 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dct-test.c

@@ -30,6 +30,7 @@
 #include <string.h>
 #include <sys/time.h>
 #include <unistd.h>
+#include <math.h>
 
 #include "dsputil.h"
 

libavcodec/fft-test.c

@@ -27,6 +27,8 @@
 #include <math.h>
 #include <unistd.h>
 #include <sys/time.h>
+#include <stdlib.h>
+#include <string.h>
 
 #undef exit