Declare msi_register() in msi.h.

The patch is especially necessary for ia64 on which most of files
emit compiler warnings like below:
  include2/asm/msi.h: In function `ia64_msi_init':
  include2/asm/msi.h:23: warning: implicit declaration of function `msi_register'
  In file included from include2/asm/machvec.h:408,
                 from include2/asm/io.h:70,
                 from include2/asm/smp.h:20,
                 from /build/rc6/source/include/linux/smp.h:22,

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>

--- linux-2.6.16-rc6-mm1.orig/include/asm-ia64/msi.h	2006-03-14 13:54:11.000000000 -0500
+++ linux-2.6.16-rc6-mm1/include/asm-ia64/msi.h	2006-03-14 14:05:26.000000000 -0500
@@ -15,6 +15,7 @@ static inline void set_intr_gate (int nr
 #define MSI_TARGET_CPU_SHIFT	4
 
 extern struct msi_ops msi_apic_ops;
+extern int msi_register(struct msi_ops *);
 
 /* default ia64 msi init routine */
 static inline int ia64_msi_init(void)