minor fix: remove a useless header

examples/ppm-downscaler/yuv-downscaler.c

@@ -19,7 +19,6 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
-#include <malloc.h>
 #include <assert.h>
 #include <stdio.h>