mandelbrot: add math.h for log2()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vsrc_mandelbrot.c

@@ -31,6 +31,7 @@
 #include "libavutil/opt.h"
 #include "libavutil/parseutils.h"
 #include <float.h>
+#include <math.h>
 
 #define SQR(a) ((a)*(a))