vfprintf etc don't take variable argument lists, but have one
parameter of type va_list that points to a variable argument list.  since you're
outside the APE environment (or it would be using /sys/include/ape/stdio.h)
you need to include <u.h> before <stdio.h>, as for nearly any
other Plan 9 include file (despite the comment to the contrary in
intro(2)).  in practice, you'll typically also need <libc.h> for any real program
(eg, to declare exits).