Converts a pointer to a gint to a hash value.
It can be passed to HashTable.new as the hash_func parameter,
when using non-NULL pointers to integer values as keys in a glib.HashTable
Note that this function acts on pointers to gint, not on gint
directly: if your hash table's keys are of the form
GINT_TO_POINTER (n), use g_direct_hash() instead.
a pointer to a gint key
a hash value corresponding to the key.