fix missing ctypes function signatures

These missing signatures were causing OSX to truncate 64-bit pointers
to 32-bit pointers when calling the missing methods, causing segfaults

python/olm/account.py

@@ -41,6 +41,7 @@ account_function(lib.olm_account_one_time_keys_length)
 account_function(lib.olm_account_one_time_keys, c_void_p, c_size_t)
 account_function(lib.olm_account_mark_keys_as_published)
 account_function(lib.olm_account_max_number_of_one_time_keys)
+account_function(lib.olm_pickle_account_length)
 account_function(
     lib.olm_account_generate_one_time_keys_random_length,
     c_size_t

python/olm/session.py

@@ -58,6 +58,7 @@ session_function(
     c_void_p, c_size_t,  # Identity Key
     c_void_p, c_size_t,  # Pre Key Message
 )
+session_function(lib.olm_pickle_session_length)
 session_function(lib.olm_encrypt_message_type)
 session_function(lib.olm_encrypt_random_length)
 session_function(lib.olm_encrypt_message_length, c_size_t)