We consider the linear growth-fragmentation equation arising in the modelling of cell
division or polymerisation processes. For constant coefficients, we prove that the dynamics converges
to the steady state with an exponential rate. The control on the initial data uses an elaborate $L1$-norm
that seems to be necessary. It also reflects the main idea of the proof, which is to use an
anti-derivative of the solution. The main technical difficulty is related to the entropy dissipation
rate, which is too weak to produce a Poincaré inequality.