src/sage/functions/piecewise.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/functions/piecewise.py

@@ -827,6 +827,7 @@ def integral(self, parameters, variable, x=None, a=None, b=None, definite=False,
 
             Check that the algorithm keyword can be used::
 
+                sage: # needs sage.libs.giac
                 sage: ex = piecewise([([0, 1], 1), ((1, oo), 1/x**2)])
                 sage: integral(ex, x, 0, 100, algorithm='giac')
                 199/100