addtags: Fix go vet errors

With newer go versions, `go vet` reports:
```
cmd/addtags/main.go:19:25: non-constant format string in call to fmt.Fprintf
cmd/addtags/main.go:24:25: non-constant format string in call to fmt.Fprintf
```
This happens because of the use of [1:] to remove an unwanted `\n` at
the beginning of the string.
This commit switches to `Fprint` instead of `Fprintf` as there are no
substitutions to be made in the string.

Signed-off-by: Christophe Fergeau <[email protected]>

Author: cfergeau

cmd/addtags/main.go

@@ -16,12 +16,12 @@ var (
 )
 
 func usage() {
-	fmt.Fprintf(os.Stderr, `
+	fmt.Fprint(os.Stderr, `
 usage: addtags -tags <build tags to apply> -file FILE <subcommand args...>
 `[1:])
 
 	flag.PrintDefaults()
-	fmt.Fprintf(os.Stderr, `
+	fmt.Fprint(os.Stderr, `
 It is intended for use with 'go generate', so it also runs a subcommand,
 which presumably creates the file.
 Sample usage: