x86, asmlinkage, xen: Fix type of NMI

LTO requires consistent types of symbols over all files.

So "nmi" cannot be declared as a char [] here, need to use the
correct function type.

Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1382458079-24450-8-git-send-email-andi@firstfloor.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/xen/setup.c

@@ -34,7 +34,7 @@
 extern const char xen_hypervisor_callback[];
 extern const char xen_failsafe_callback[];
 #ifdef CONFIG_X86_64
-extern const char nmi[];
+extern asmlinkage void nmi(void);
 #endif
 extern void xen_sysenter_target(void);
 extern void xen_syscall_target(void);
@@ -559,7 +559,7 @@ void xen_enable_syscall(void)
 void xen_enable_nmi(void)
 {
 #ifdef CONFIG_X86_64
-	if (register_callback(CALLBACKTYPE_nmi, nmi))
+	if (register_callback(CALLBACKTYPE_nmi, (char *)nmi))
 		BUG();
 #endif
 }