monster: add BUGFIX comment for monstkills

The size of the monstkills array is unnecessarily large,
as it maps from monster type to number of kills, and the
monster type enum goes from 0 to NUM_MTYPES-1.

Source/monster.cpp

@@ -4,6 +4,7 @@
 // Tracks which missile files are already loaded
 int MissileFileFlag;
 
+// BUGFIX: replace monstkills[MAXMONSTERS] with monstkills[NUM_MTYPES].
 int monstkills[MAXMONSTERS];
 int monstactive[MAXMONSTERS];
 int nummonsters;