bug#52791: nextcloud-client not building on master

[Nicholas von Klitzing]

Hello Guix,

nextcloud-client has been broken in recent updates to guix and is no longer 
building:

https://ci.guix.gnu.org/build/238676/details

All the errors are in the form of
```
/tmp/guix-build-nextcloud-client-3.1.3.drv-0/build/src/gui/nextcloud_autogen/YCDLW3T4OG/qrc_theme.cpp
In file included from 
/gnu/store/qqs98rxwjrji6aaf6dqwp7q4m545g2sn-glib-2.70.0/include/glib-2.0/glib/glib-typeof.h:39,
                 from 
/gnu/store/qqs98rxwjrji6aaf6dqwp7q4m545g2sn-glib-2.70.0/include/glib-2.0/glib/gatomic.h:28,
                 from 
/gnu/store/qqs98rxwjrji6aaf6dqwp7q4m545g2sn-glib-2.70.0/include/glib-2.0/glib/gthread.h:32,
                 from 
/gnu/store/qqs98rxwjrji6aaf6dqwp7q4m545g2sn-glib-2.70.0/include/glib-2.0/glib/gasyncqueue.h:32,
                 from 
/gnu/store/qqs98rxwjrji6aaf6dqwp7q4m545g2sn-glib-2.70.0/include/glib-2.0/glib.h:32,
                 from 
/tmp/guix-build-nextcloud-client-3.1.3.drv-0/source/src/gui/cloudproviders/cloudprovidermanager.cpp:16:
/gnu/store/vakvgvrb839igv16jkif4lmx11d25jqb-gcc-10.3.0/include/c++/type_traits:56:3:
 error: template with C linkage
   56 |   template<typename _Tp, _Tp __v>
      |   ^~~~~~~~
/tmp/guix-build-nextcloud-client-3.1.3.drv-0/source/src/gui/cloudproviders/cloudprovidermanager.cpp:15:1:
 note: ?extern "C"? linkage started here
   15 | extern "C" {
      | ^~~~~~~~~~
```

They see to be directly related to an issue patched in more recent versions of 
Nextcloud Desktop:

https://github.com/nextcloud/desktop/pull/2972

Kind regards,
Nicholas von Klitzing