Add missing imports

Fix issue #75

ffmpeg_linux_test.go

@@ -1,6 +1,8 @@
 package ffmpeg_go
 
 import (
+	"os/exec"
+	"syscall"
 	"testing"
 
 	"github.com/stretchr/testify/assert"

run_linux.go

@@ -5,8 +5,10 @@ import (
 	"errors"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strconv"
+	"syscall"
 
 	"github.com/u2takey/go-utils/rand"
 )