Since $X$ takes values in $\{ 0, 1 \}$, then $e^{i t X}$ takes values in $\{ e^{i t \cdot 1}, e^{i t \cdot 0} \} = \{ e^{i t}, 1 \}$. Applying
R1814: Expectation of discrete random euclidean real number, one then has
\begin{equation}
\begin{split}
\mathbb{E}(e^{i t X}) & = e^{i t \cdot 1} \mathbb{P}(X = 1) + e^{i t \cdot 0} \mathbb{P}(X = 0) \\
& = e^{i t} \theta + 1 \cdot (1 - \theta) \\
& = \theta e^{i t} + 1 - \theta \\
\end{split}
\end{equation}
$\square$