xen/p2m: Make debug/xen/mmu/p2m visible again.

We dropped a lot of the MMU debugfs in favour of using
tracing API - but there is one which just provides
mostly static information that was made invisible by this change.

Bring it back.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

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

@@ -53,9 +53,6 @@ extern int m2p_remove_override(struct page *page, bool clear_pte);
 extern struct page *m2p_find_override(unsigned long mfn);
 extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
 
-#ifdef CONFIG_XEN_DEBUG_FS
-extern int p2m_dump_show(struct seq_file *m, void *v);
-#endif
 static inline unsigned long pfn_to_mfn(unsigned long pfn)
 {
 	unsigned long mfn;