x86/setup: Make setup.h header self contained

The header uses rangeset structure pointer.
Forward declare the structure.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/include/asm/setup.h

@@ -43,6 +43,7 @@ void *bootstrap_map_bm(const struct boot_module *bm);
 void *bootstrap_map(const module_t *mod);
 void bootstrap_unmap(void);
 
+struct rangeset;
 int remove_xen_ranges(struct rangeset *r);
 
 int cf_check stub_selftest(void);