x86/setup: drop stale declarations of __bss_{start,end}[]

There are no references anymore as of c9a4a1c419 ("x86/layout: Correct
Xen's idea of its own memory layout"). For what's left, switch to
"unsigned char" as here we're not dealing with strings of any kind.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/setup.c

@@ -305,7 +305,7 @@ void __init discard_initial_images(void)
     initial_images = NULL;
 }
 
-extern char __init_begin[], __init_end[], __bss_start[], __bss_end[];
+extern unsigned char __init_begin[], __init_end[];
 
 static void __init init_idle_domain(void)
 {