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 <minux.ma@gmail.com>