(cf. LO's configure.ac, needed for external/firebird on non-x86[-64]), but
org.freedesktop.Sdk//18.08 apparently doesn't provide it (while
org.freedesktop.Sdk//1.6 apparently did)

(cf. <https://github.com/flathub/org.libreoffice.LibreOffice/pull/67/commits/
b2762523a4678bedd55c11db232868cbaf9c7bdd> "aarch64 and arm need libatomic_ops")

Change-Id: I6eb060251ce85bfd399b72d9c90a93497a3a9029
Reviewed-on: https://gerrit.libreoffice.org/66987
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>