vsprintf: Include stdarg for va_list

vsprintf.h doesn't include the stdarg.h file, which means that it relies on
the files that include vsprintf.h to include stdarg.h as well.

Add an explicit include to avoid build errors when simply including that
file.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>