When using GCC on AMD64 and PPC, ECPGget_variable() takes a va_list *, not

When using GCC on AMD64 and PPC, ECPGget_variable() takes a va_list *, not
a va_list. Christof Petig's previous patch made this change, but neglected
to update ecpglib/descriptor.c, resulting in a compiler warning (and a
likely runtime crash) on AMD64 and PPC.