Fix Compile problem with gcc 10.2

PS : With flatpak 1.10.2 we even don't need extern part until we require that version this patch needed (for gcc10.2 users )

Without this patch it couldn't compile and gives this error :

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 /usr/include/glib-2.0/gobject/gbinding.h:28, from /usr/include/glib-2.0/glib-object.h:22, from /usr/include/glib-2.0/gio/gioenums.h:28, from /usr/include/glib-2.0/gio/giotypes.h:28, from /usr/include/glib-2.0/gio/gio.h:26, from /usr/include/flatpak/flatpak.h:25, from /home/kde-dev/kde/src/plasma/discover/libdiscover/backends/FlatpakBackend/FlatpakTransactionThread.h:18, from /home/kde-dev/kde/src/plasma/discover/libdiscover/backends/FlatpakBackend/FlatpakTransactionThread.cpp:7: /usr/include/c++/10.2.0/type_traits:2801:3: error: template with C linkage