Found it after searching for some time:
[ideRootPath, ~] = codertarget.arduinobase.internal.getArduinoIDERoot('libraries');
addIncludePaths(buildInfo, fullfile(ideRootPath, 'libraries', 'Servo', 'src'));
srcFilePath = fullfile(ideRootPath, 'libraries', 'Servo', 'src', 'avr');
fileNameToAdd = {'Servo.cpp'};
addSourceFiles(buildInfo, fileNameToAdd, srcFilePath);