kbuild/genksyms: handle va_list type

genksyms currently does not handle va_list. Add the __builtin_va_list
keyword as a type. This reduces the amount of syntax errors thrown,
but so far no export symbol has a type with a va_list argument, so
there is currently no bug in the end result.

Note: this patch does not regenerate shipped parser files.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>