Fix member assignment in Arena to fit C89 standard

becs.h

@@ -117,11 +117,10 @@ static bool BECS_SignatureMatches(BECS_Signature entitySig, BECS_Signature query
 BECS_Arena BECS_CreateArena(void *memory, size_t capacity)
 {
     uintptr_t baseOffset = 64 - ((uintptr_t)memory % 64);
-    BECS_Arena arena = {
-        baseOffset,
-        capacity,
-        memory
-    };
+    BECS_Arena arena = {};
+    arena.nextAllocation = baseOffset;
+    arena.capacity = capacity;
+    arena.memory = memory;
     return arena;
 }