include/plan9/libc.h: fix Plan 9 build
include/plan9/libc.h: fix Plan 9 build

Add missing getgoextlinkenabled(void) declaration
in Plan 9 libc.h. This function was added as part
of CL #8183043.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/8191043

Committer: Shenghou Ma <b563c57947dc20eb81988e3e3c22786764c14ca8@gmail.com>

mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go