libdnf.h: Remove overall extern "C"
libdnf.h: Remove overall extern "C"

See https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1935

The individual header files already do this, which is
the right thing to do.  Otherwise we force on C linkage
even for external header files like the ones from GLib,
but they may want to use C++ features if they detect
they're being used in C++.

(In fact GLib offers `G_BEGIN_DECLS/G_END_DECLS`
 which is a nice macro wrapper for this, but we can switch
 to that later)

rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf