x86: fix warning in "x86: clean up vSMP detection"

The function detect_vsmp_box is a void function in the PCI case.
Change the !PCI stub to void too.
Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/vsmp_64.c

@@ -133,7 +133,7 @@ int is_vsmp_box(void)
 	}
 }
 #else
-static int __init detect_vsmp_box(void)
+static void __init detect_vsmp_box(void)
 {
 }
 int is_vsmp_box(void)