arm/keyhandler: declare struct cpu_user_regs

in the keyhandler.h file. Otherwise on ARM builds if we
just use the keyhandler file - the compile will fail.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

xen/include/xen/keyhandler.h

@@ -24,6 +24,7 @@ typedef void (keyhandler_fn_t)(unsigned char key);
  *
  * Called in hardirq context with interrupts disabled.
  */
+struct cpu_user_regs;
 typedef void (irq_keyhandler_fn_t)(unsigned char key,
                                    struct cpu_user_regs *regs);