src/sage/calculus/calculus.py: add "needs" for libgiac integration

One example in this file integrates with algorithm='giac', which can
fail if sage.libs.giac is not available to perform the integration.

Author: orlitzky

src/sage/calculus/calculus.py

@@ -387,6 +387,7 @@
 Ensure that :issue:`25626` is fixed. As the form of the answer is dependent of
 the giac version, we simplify it (see :issue:`34037`). ::
 
+    sage: # needs sage.libs.giac
     sage: t = SR.var('t')
     sage: integrate(exp(t)/(t + 1)^2, t, algorithm='giac').full_simplify()
     ((t + 1)*Ei(t + 1) - e^(t + 1))/(t*e + e)