Broke the build on Ubuntu 18.04:

In file included from /home/tinderbox/core-master/workdir/UnoApiHeadersTarget/udkapi/normal/com/sun/star/uno/XInterface.hpp:10:0,
                 from /home/tinderbox/core-master/workdir/UnoApiHeadersTarget/udkapi/normal/com/sun/star/uno/Exception.hpp:8,
                 from /home/tinderbox/core-master/workdir/UnoApiHeadersTarget/udkapi/normal/com/sun/star/uno/RuntimeException.hpp:8,
                 from /home/tinderbox/core-master/include/com/sun/star/uno/Any.hxx:37,
                 from /home/tinderbox/core-master/include/com/sun/star/uno/genfunc.hxx:27,
                 from /home/tinderbox/core-master/include/com/sun/star/uno/Sequence.hxx:35,
                 from /home/tinderbox/core-master/include/svl/sigstruct.hxx:27,
                 from /home/tinderbox/core-master/xmlsecurity/inc/xsecctl.hxx:23,
                 from /home/tinderbox/core-master/xmlsecurity/source/helper/xsecctl.cxx:22:
/home/tinderbox/core-master/include/com/sun/star/uno/Reference.hxx: In instantiation of ‘static com::sun::star::uno::XInterface* com::sun::star::uno::Reference< <template-parameter-1-1> >::iquery(com::sun::star::uno::XInterface*) [with interface_type = com::sun::star::xml::crypto::XXMLSignature]’:
/home/tinderbox/core-master/include/com/sun/star/uno/Reference.hxx:279:42:   required from ‘bool com::sun::star::uno::Reference< <template-parameter-1-1> >::set(const com::sun::star::uno::BaseReference&, com::sun::star::uno::UnoReference_Query) [with interface_type = com::sun::star::xml::crypto::XXMLSignature]’
/home/tinderbox/core-master/xmlsecurity/source/helper/xsecctl.cxx:190:124:   required from here
/home/tinderbox/core-master/include/com/sun/star/uno/Reference.hxx:70:73: error: incomplete type ‘com::sun::star::xml::crypto::XXMLSignature’ used in nested name specifier
     return BaseReference::iquery(pInterface, interface_type::static_type());
                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/home/tinderbox/core-master/solenv/gbuild/LinkTarget.mk:290: recipe for target '/home/tinderbox/core-master/workdir/CxxObject/xmlsecurity/source/helper/xsecctl.o' failed
make[1]: *** [/home/tinderbox/core-master/workdir/CxxObject/xmlsecurity/source/helper/xsecctl.o] Error 1

Change-Id: I864da5bd6d163a3de35280bff367e6b51aced23d
Reviewed-on: https://gerrit.libreoffice.org/73035Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins