Annoying little bonus: Fix ghostly bot bodies in singleplayer Prisonbreak from command line not getting discarded

This happens ALL THE TIME while I'm testing CDSS1 remaster direct from ZB, and this is essentially the CDSS1 remaster branch

src/d_main.c

@@ -1969,6 +1969,8 @@ void D_SRB2Main(void)
 						grandprixinfo.eventmode = GPEVENT_SPECIAL;
 					else if (gametype != GT_RACE)
 						grandprixinfo.eventmode = GPEVENT_BONUS;
+
+					multiplayer = true;
 				}
 
 				G_SetUsedCheats();