update stdio.h for including stdarg.h

it seems that including stdarg.h is needed for defining va_list
environment is hpux 11.31(ia64) with gcc 4.7.1.

without stdarg.h, I got compilation error like this.
  "error: 'va_list' undeclared (first use in this function)"

I checked with gcc -E, then I noticed that __va_list is defined but va_list is not.
  "typedef __gnuc_va_list __va_list;"

with including stdarg.h, va_list is defined.
  "typedef __gnuc_va_list va_list;"