powerpc/mm: Declare arch_report_meminfo() prototype.

It fixes this W=1 compile error :

../arch/powerpc/mm/book3s64/pgtable.c:411:6: error: no previous prototype for ‘arch_report_meminfo’ [-Werror=missing-prototypes]
  411 | void arch_report_meminfo(struct seq_file *m)
      |      ^~~~~~~~~~~~~~~~~~~
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-17-clg@kaod.org

arch/powerpc/include/asm/pgtable.h

@@ -162,6 +162,9 @@ static inline bool is_ioremap_addr(const void *x)
 
 	return addr >= IOREMAP_BASE && addr < IOREMAP_END;
 }
+
+struct seq_file;
+void arch_report_meminfo(struct seq_file *m);
 #endif /* CONFIG_PPC64 */
 
 #endif /* __ASSEMBLY__ */