arena: move kb macro

src/utils/arena.c

@@ -4,8 +4,6 @@
 #include <stdlib.h>
 #include <string.h>
 
-#define KB(x) x * 1024
-
 void arena_init(Arena *a, size_t size) {
   *a = (Arena){.buffer = malloc(size), .size = size, .offset = 0};
   if (a->buffer == NULL)

src/utils/arena.h

@@ -3,6 +3,8 @@
 
 #include <stddef.h>
 
+#define KB(x) x * 1024
+
 typedef struct {
   char *buffer;
   size_t size;