Automatically calculate REMATCH_TABLE_ENTRIES

Uses the standard C way of calculating the length of an enum.

include/battle_setup.h

@@ -4,7 +4,6 @@
 #include "gym_leader_rematch.h"
 
 #define REMATCHES_COUNT 5
-#define REMATCH_TABLE_ENTRIES 78
 #define REMATCH_ELITE_FOUR_ENTRIES REMATCH_SIDNEY
 
 struct RematchTrainer

include/gym_leader_rematch.h

@@ -81,7 +81,11 @@ enum {
     REMATCH_PHOEBE,
     REMATCH_GLACIA,
     REMATCH_DRAKE,
-    REMATCH_WALLACE
+    REMATCH_WALLACE,
+
+    // The total number of rematch entries. This must be the last element
+    // in the enum.
+    REMATCH_TABLE_ENTRIES
 };
 
 void UpdateGymLeaderRematch(void);