Use usleep rather than nanosleep

kitty/disk-cache.h

@@ -13,6 +13,7 @@ bool add_to_disk_cache(PyObject *self, const void *key, size_t key_sz, const voi
 bool remove_from_disk_cache(PyObject *self_, const void *key, size_t key_sz);
 void* read_from_disk_cache(PyObject *self_, const void *key, size_t key_sz, void*(allocator)(void*, size_t), void*);
 bool disk_cache_wait_for_write(PyObject *self, monotonic_t timeout);
+size_t disk_cache_size_on_disk(PyObject *self);
 
 static inline void* disk_cache_malloc_allocator(void *x, size_t sz) {
     *((size_t*)x) = sz;