add orbitTimeSincePeriapsis prototype

src/orbits.cpp

@@ -266,7 +266,10 @@ orbitClampAngle(double theta)
 }
 
 double
-orbitGetTimeOfFlight(system_2body sys, double theta_0, double theta_1)
+orbitTimeSincePeriapsis(system_2body sys, double theta);
+
+double
+orbitGetTimeOfFlight(system_2body sys, double theta_begin, double theta_end)
 {
 	double e = sys.ep.e;