all: add go directive to go.mod

Go 1.12 adds this line automatically, so let's just go with it.

Change-Id: I0ac156a58f0f107e019a8c06ebf189b819a3651d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/165751
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go.mod

@@ -1 +1,3 @@
 module golang.org/x/sys
+
+go 1.12