mbox: Forward-declare struct mbox_context

Avoid compilation errors such as:

    $ make
    make  all-am
    make[1]: Entering directory '/home/andrew/src/openbmc/phosphor-mboxd'
      CC       mboxd-common.o
      CC       mboxd-mboxd_dbus.o
      CC       mboxd-mboxd_lpc.o
      CC       mboxd-mboxd_msg.o
      CC       mboxd-mboxd_windows.o
      CC       mboxd-mtd.o
      CC       mboxd-mboxd_flash.o
      CC       mboxctl-mboxctl.o
      CC       mboxd-mboxd_lpc_reset.o
    In file included from mboxd_dbus.c:28:0:
    mbox.h:133:42: error: ‘struct mbox_context’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
     typedef int (*mboxd_mbox_handler)(struct mbox_context *, union mbox_regs *,
                                              ^~~~~~~~~~~~
    cc1: all warnings being treated as errors
    Makefile:2201: recipe for target 'mboxd-mboxd_dbus.o' failed
    make[1]: *** [mboxd-mboxd_dbus.o] Error 1
    make[1]: *** Waiting for unfinished jobs....

Change-Id: Ie59a7b6a0831dc509b133fe35a2107f802afb49c
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mbox.h b/mbox.h
index 7ae002b..19d0a3f 100644
--- a/mbox.h
+++ b/mbox.h
@@ -130,6 +130,8 @@
 	struct mbox_msg msg;
 };
 
+struct mbox_context;
+
 typedef int (*mboxd_mbox_handler)(struct mbox_context *, union mbox_regs *,
 				  struct mbox_msg *);