Add missing import to promtool, fix build (#4395)

Sorry, I used GitHub's web-based merge-conflict-resolution editor on
https://github.com/prometheus/prometheus/pull/4308 and it didn't show me
test errors afterwards, but maybe they didn't run again or I should have
waited or something.

Signed-off-by: Julius Volz <julius.volz@gmail.com>

cmd/promtool/main.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"math"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"