Fix decision procedure runtime computation

Follow-up for 2dc9fcd.

Author: tautschnig

regression/cbmc/Failing_Assert1/test.desc

@@ -4,5 +4,6 @@ main.c
 ^EXIT=10$
 ^SIGNAL=0$
 ^VERIFICATION FAILED$
+^Runtime decision procedure: [0-9]+(\.[0-9]+)?s$
 --
 ^warning: ignoring

src/cbmc/bmc.cpp

@@ -170,7 +170,7 @@ bmct::run_decision_procedure(prop_convt &prop_conv)
   {
     auto solver_stop = std::chrono::steady_clock::now();
     status() << "Runtime decision procedure: "
-             << std::chrono::duration<double>(solver_start-solver_stop).count()
+             << std::chrono::duration<double>(solver_stop-solver_start).count()
              << "s" << eom;
   }