runtime: always declare nanotime in Go

For libgo it's always defined in C.

Updates https://gcc.gnu.org/PR86535

Change-Id: I8d7d5fcb55cee60b91007cec1f3a6e8aae104f10
Reviewed-on: https://go-review.googlesource.com/c/163743
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed