x86: move stack_start to smp.h

voyager would conflict with it, but the types are ultimately
compatible. So remove the extern definition from voyager_smp.c
in favour of the common one

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/asm-x86/smp.h b/include/asm-x86/smp.h
index d02e6ea..78ef16d 100644
--- a/include/asm-x86/smp.h
+++ b/include/asm-x86/smp.h
@@ -28,6 +28,13 @@
 extern const unsigned char trampoline_end  [];
 extern unsigned char *trampoline_base;
 
+/* Static state in head.S used to set up a CPU */
+extern struct {
+	void *sp;
+	unsigned short ss;
+} stack_start;
+
+
 struct smp_ops {
 	void (*smp_prepare_boot_cpu)(void);
 	void (*smp_prepare_cpus)(unsigned max_cpus);