cmd/vet: Incorrect warning for build tag in string.

[ryanslade]

go vet does a simple scan of the code looking for lines starting with "// +build", it is possible for this to appear in valid code. For example:

package main

const template = '
// +build example

package thing

func main() {
}'

func main() {
    // Whatever
}

[rsc]

It's a warning. You can ignore it. Trying to split this hair is probably not worthwhile.