Move notification out of block locking associated mutex in Gravity.cpp

I don't know how it ended up inside, I wanted it outside. For the record,
signalling the CV while the associated mutex is locked is a pessimisation,
it just yields predictable scheduling.

src/simulation/Gravity.cpp

@@ -125,12 +125,12 @@ void Gravity::gravity_update_async()
 					signal_grav = true;
 				}
 			}
+		}
 
 		if (signal_grav)
 		{
 			gravcv.notify_one();
 		}
-		}
 		//Apply the gravity mask
 		membwand(gravy, gravmask, (XRES/CELL)*(YRES/CELL)*sizeof(float), (XRES/CELL)*(YRES/CELL)*sizeof(unsigned));
 		membwand(gravx, gravmask, (XRES/CELL)*(YRES/CELL)*sizeof(float), (XRES/CELL)*(YRES/CELL)*sizeof(unsigned));