[POWERPC] Add a declaration for xmon()

Usually we call xmon() via debugger(), so this could be static.
Sometimes when debugging it's nice to be able to call xmon()
directly though, so add a declaration.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/xmon.h

@@ -15,6 +15,8 @@
 #ifdef CONFIG_XMON
 extern void xmon_setup(void);
 extern void xmon_register_spus(struct list_head *list);
+struct pt_regs;
+extern int xmon(struct pt_regs *excp);
 #else
 static inline void xmon_setup(void) { };
 static inline void xmon_register_spus(struct list_head *list) { };