Update code to work on mac

ppm.c

@@ -10,7 +10,8 @@ void write_p6_ppm(const char *filepath, u64 width, u64 height, Pixel *data) {
   }
 
   char header[1024] = {0};
-  sprintf(header, "P6\n%lu %lu\n255\n", width, height);
+  sprintf(header, "P6\n%llu %llu\n255\n", (unsigned long long)width,
+          (unsigned long long)height);
 
   u64 length = strlen(header);
   fwrite(&header, length, 1, fp);