Drop use of libgsystem, move single API call into shell-global.c

See https://mail.gnome.org/archives/desktop-devel-list/2015-February/msg00121.html

We presently only indirectly link to it via this one call; until GLib
gains structured logging, simply import the code to wrap the journal
logging in an introspectable API into Shell.

https://bugzilla.gnome.org/show_bug.cgi?id=744457

src/shell-global.h

@@ -124,6 +124,9 @@ void     shell_global_init_xdnd                 (ShellGlobal  *global);
 
 void     shell_global_reexec_self               (ShellGlobal  *global);
 
+void     shell_global_log_structured            (const char *message,
+                                                 const char *const *keys);
+
 const char *     shell_global_get_session_mode  (ShellGlobal  *global);
 
 void     shell_global_set_runtime_state         (ShellGlobal  *global,