log: Unbreak windows support

Add the missing define.

libavutil/log.c

@@ -50,6 +50,7 @@ static int16_t background, attr_orig;
 static HANDLE con;
 #define set_color(x)  SetConsoleTextAttribute(con, background | color[x])
 #define reset_color() SetConsoleTextAttribute(con, attr_orig)
+#define print_256color(x)
 #else
 static const uint8_t color[] = { 0x41, 0x41, 0x11, 0x03, 9, 0x02, 0x06 };
 #define set_color(x)  fprintf(stderr, "\033[%d;3%dm", color[x] >> 4, color[x]&15)