Do no include glib.h with extern C

[ivaradi]

Compilation fails on Hirsute with errors like:

In file included from /usr/include/glib-2.0/glib/gatomic.h:31,
                 from /usr/include/glib-2.0/glib/gthread.h:32,
                 from /usr/include/glib-2.0/glib/gasyncqueue.h:32,
                 from /usr/include/glib-2.0/glib.h:32,
                 from /<<PKGBUILDDIR>>/src/gui/cloudproviders/cloudprovidermanager.cpp:16:
/usr/include/c++/10/type_traits:56:3: error: template with C linkage
   56 |   template<typename _Tp, _Tp __v>
      |   ^~~~~~~~
/<<PKGBUILDDIR>>/src/gui/cloudproviders/cloudprovidermanager.cpp:15:1: note: ‘extern "C"’ linkage started here
   15 | extern "C" {
      | ^~~~~~~~~~

This is due to glib/gatomic.h now including <type_traits> when being compiled with C++. I think glib.h and similar headers can be safely included into C++ code without extern "C" nowadays. Hence this patch removes it from cloudprovidermanager.cpp.

[nextcloud-desktop-bot]

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[FlexW]

Thank you for that fix. According to here https://gitlab.gnome.org/GNOME/glib/-/issues/2331 it is even not supported to compile with extern C.