Assuming you’re building against the 8.x SDK, and you have set the deployment target to 8.x or higher, that’ll be enough.
However, I’d imagine you set the deployment target the wrong place in Xcode. You’ll need to set it under your Project and not under your Target. Alternatively, if you have a .xcconfig file, you can change it by setting
IPHONEOS_DEPLOYMENT_TARGET = 8.1
If you don’t have a .xcconfig, you can verify the deployment target, by opening your project.pbxproj file, and search for IPHONEOS_DEPLOYMENT_TARGET, and check if it’s set to your expected value.