Corrected PK Mapflag assignment (#4165)

* Fixes an issue with different player versus player mapflags not being checked before applying the PK mapflag.
Thanks to @LiamKarlMitchell!

src/map/map.cpp

@@ -3660,7 +3660,7 @@ void map_flags_init(void){
 			continue;
 
 		// adjustments
-		if( battle_config.pk_mode && !mapdata->flag[MF_PVP] )
+		if( battle_config.pk_mode && !mapdata_flag_vs2(mapdata) )
 			mapdata->flag[MF_PVP] = true; // make all maps pvp for pk_mode [Valaris]
 	}
 }