utils: fix compilation on Darwin

Although Docker daemon does not work on Darwin, the API client will have
to work. That said, I'm fixing the compilation of the package on Darwin.

utils/uname_darwin.go

@@ -2,9 +2,12 @@ package utils
 
 import (
 	"errors"
-	"syscall"
 )
 
-func uname() (*syscall.Utsname, error) {
+type Utsname struct {
+	Release [65]byte
+}
+
+func uname() (*Utsname, error) {
 	return nil, errors.New("Kernel version detection is not available on darwin")
 }