samba4: updated to 4.15.6

Changes since 4.15.5
--------------------
* BUG 14169: Renaming file on DFS root fails with
  NT_STATUS_OBJECT_PATH_NOT_FOUND.
* BUG 14737: Samba does not response STATUS_INVALID_PARAMETER when opening 2
  objects with same lease key.
* BUG 14938: NT error code is not set when overwriting a file during rename
  in libsmbclient.
* BUG 14996: Fix ldap simple bind with TLS auditing.
* BUG 14674: net ads info shows LDAP Server: 0.0.0.0 depending on contacted
  server.
* BUG 14979: Problem when winbind renews Kerberos.
* BUG 8691: pam_winbind will not allow gdm login if password about to expire.
* BUG 14971: virusfilter_vfs_openat: Not scanned: Directory or special file.
* BUG 13631: DFS fix for AIX broken.
* BUG 14974: Solaris and AIX acl modules: wrong function arguments.
* BUG 7239: Function aixacl_sys_acl_get_file not declared / coredump.
* BUG 14900: Regression: Samba 4.15.2 on macOS segfaults intermittently
  during strcpy in tdbsam_getsampwnam.
* BUG 14989: Fix a use-after-free in SMB1 server.
* BUG 14968: smb2_signing_decrypt_pdu() may not decrypt with
  gnutls_aead_cipher_decrypt() from gnutls before 3.5.2.
* BUG 14984: changing the machine password against an RODC likely destroys
  the domain join.
* BUG 14993: authsam_make_user_info_dc() steals memory from its struct
  ldb_message *msg argument.
* BUG 14995: Use Heimdal 8.0 (pre) rather than an earlier snapshot.
* BUG 14967: Samba autorid fails to map AD users if id rangesize fits in the
  id range only once.

(adam)