Since the bonus came from your employer it represents compensation, which is taxable for income as well as social security tax purposes. However, you are not self-employed so technically are not subject to self-employment tax, even though it goes to social security just like if it was subtracted from wages.
You can report the income as wage income not reported on a W-2 form. You can do this in TurboTax by entering the bonus in the Wages and Income section, then Less Common Income, then Miscellaneous Income, 1099-A, 1099-C. Choose the category Other Income not already reported on a Form W-2 or Form 1099. This will result in the social security tax being added to your income tax, so you won't need to report the income as self-employment income.