go.mod: add go 1.12 line

ulikunitz · ulikunitz · commit 818631ae61b0 · 2020-02-24T09:37:27.000+01:00
Without the line go.mod will be changed by the go command.
diff --git a/go.mod b/go.mod
@@ -1 +1,3 @@
 module github.com/ulikunitz/xz
+
+go 1.12

Original file line number	Diff line number	Diff line change
`@@ -1 +1,3 @@`
`1`	`1`	`module github.com/ulikunitz/xz`
	`2`	`+`
	`3`	`+go 1.12`