Missing scanf %f

The program `xscreensaver-gfx` calls sscanf with `" %f %c"`. However, `%f` is, as far as I can tell, not implemented in [`do_scanf`](https://github.com/managarm/mlibc/blob/f3e45cac82dcc1a90d8d1629e1b3e75d4fffe9df/options/ansi/generic/stdio-stubs.cpp#L351). This causes the program to segfault.