[desktop-kit] app-office/gnucash-5.11 doesn't compile after libxml2 upgrade

```
[101/834] Building CXX object libgnucash/backend/xml/CMakeFiles/gnc-backend-xml-utils.dir/gnc-budget-xml-v2.cpp.o
FAILED: [code=1] libgnucash/backend/xml/CMakeFiles/gnc-backend-xml-utils.dir/gnc-budget-xml-v2.cpp.o
/usr/bin/x86_64-pc-linux-gnu-g++ -DBOOST_DATE_TIME_DYN_LINK -DBOOST_DATE_TIME_NO_LIB -DBOOST_REGEX_DYN_LINK -DBOOST_REGEX_NO_LIB -DGLIB_VERSION_MAX_ALLOWED=GLIB_VERSION_2_56 -DGLIB_VERSION_MIN_REQUIRED=GLIB_VERSION_2_56 -DG_LOG_DOMAIN=\"gnc.backend.xml\" -DHAVE_CONFIG_H -DHAVE_GUILE22 -DU_SHOW_CPLUSPLUS_API=0 -D_GNU_SOURCE -Dgnc_backend_xml_utils_EXPORTS -I/usr/include/libxml2 -I/var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/backend/xml -I/var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/engine -I/var/tmp/portage/app-office/gnucash-5.11/work/gnucash-5.11_build/common -I/var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/common -I/var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/borrowed/libc -I/var/tmp/portage/app-office/gnucash-5.11/work/gnucash-5.11_build/libgnucash/core-utils -I/var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/core-utils -isystem /usr/include/glib-2.0 -isystem /usr/lib64/glib-2.0/include -Werror -Wall -Wmissing-declarations -mtune=generic -O2 -pipe -std=c++17 -fPIC -MD -MT libgnucash/backend/xml/CMakeFiles/gnc-backend-xml-utils.dir/gnc-budget-xml-v2.cpp.o -MF libgnucash/backend/xml/CMakeFiles/gnc-backend-xml-utils.dir/gnc-budget-xml-v2.cpp.o.d -o libgnucash/backend/xml/CMakeFiles/gnc-backend-xml-utils.dir/gnc-budget-xml-v2.cpp.o -c /var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/backend/xml/gnc-budget-xml-v2.cpp
In file included from /var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/backend/xml/gnc-xml-helper.h:32,
from /var/tmp/portage/app-office/gnucash-5.11/work/Gnucash-gnucash-5bedd7f/libgnucash/backend/xml/gnc-budget-xml-v2.cpp:29:
/usr/include/libxml2/libxml/SAX.h:18:4: error: #warning "libxml/SAX.h is deprecated" [-Werror=cpp]
18 | #warning "libxml/SAX.h is deprecated"
| ^~~~~~~
cc1plus: all warnings being treated as errors
[106/834] Building CXX object libgnucash/app-utils/CMakeFiles/gnc-app-utils.dir/gnc-quotes.cpp.o
ninja: build stopped: subcommand failed.
* ERROR: app-office/gnucash-5.11::desktop-kit failed (install phase):
* died running ninja install
*
* Call stack:
* ebuild.sh, line 93: Called src_install
* environment, line 3775: Called cmake_src_install
```

Start with libgnucash/backend/xml/gnc-xml-helper.h, which is included by gnc-budget-xml-v2.cpp and brings in the deprecated libxml/SAX.h header. Reproduce the failure by building app-office/gnucash-5.11 with the shown configuration, then verify the build completes without the deprecated-header error under -Werror.

Written by the indexing model from the issue text.