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>

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