For this purpose you need to have Ant plugin installed in Jenkins. If not installed then download it and install. After this you can follow these steps :
1. Go to jenkins dashboard and click on manage jenkins
2. click on global tool configuration.
3. In this section you will see Ant tab with Ant installation. You need to specify Ant name and its version. Also specify the Ant home.
4. Click on apply.
After this when you create a new job for your project follow these steps -
1. Select Ant in the Build Environment section.
2. Specify the path of the build.xml file. Click on apply.
Now you can build your project by clicking build now