Fix build with gcc 12
Fixes:
[ 126s] ../sftp/kio_sftp.cpp: In function 'constexpr bool {anonymous}::KSFTP_ISDIR(SFTPAttributesPtr&)':
[ 126s] ../sftp/kio_sftp.cpp:88:14: error: call to non-'constexpr' function 'std::unique_ptr<_Tp, _Dp>::pointer std::unique_ptr<_Tp, _Dp>::operator->() const [with _Tp = sftp_attributes_struct; _Dp = std::default_delete<sftp_attributes_struct>; pointer = sftp_attributes_struct*]'
[ 126s] 88 | return sb->type == SSH_FILEXFER_TYPE_DIRECTORY;
[ 126s] | ^~
Change suggested by @sune