计算出 `gatomicrefcount` 的初始引用计数。
此宏对于初始化结构内部的 `gatomicrefcount` 字段非常有用,例如
typedef struct {
gatomicrefcount ref_count;
char *name;
char *address;
} Person;
static const Person default_person = {
.ref_count = G_ATOMIC_REF_COUNT_INIT,
.name = "Default name",
.address = "Default address",
};