runtime/cgo: fix data declaration to be extern

Otherwise some OS X toolchains complain about the redeclaration
of libcgo_thread_start by multiple object files.  The real definition
is in util.c.

Fixes #2167.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5364045

src/pkg/runtime/cgo/libcgo.h

@@ -42,7 +42,7 @@ struct ThreadStart
  * Makes a local copy of the ThreadStart and
  * calls libcgo_sys_thread_start(ts).
  */
-void (*libcgo_thread_start)(ThreadStart *ts);
+extern void (*libcgo_thread_start)(ThreadStart *ts);
 
 /*
  * Creates the new operating system thread (OS, arch dependent).