bufsize calculation in MessageBoxTimeoutIndirectW is wrong

file: lib/user32/windows/message.c

function: MessageBoxTimeoutIndirectW

the calculation for bufsize neglects some of the data being stuffed
into the buffer. For now we are adding an additional 1024 bytes to
compensate, but eventually the calculation should be corrected.

Recommended that even when bufsize calculation is fixed, that we
still leave a small "oops" buffer in, because someone may break
it in the future.