riscv: Fix undefined reference to vmemmap_populate_basepages

Using CONFIG_SPARSEMEM_VMEMMAP instead of CONFIG_SPARSEMEM to fix
following build issue.

  riscv64-linux-ld: arch/riscv/mm/init.o: in function 'vmemmap_populate':
  init.c:(.meminit.text+0x8): undefined reference to 'vmemmap_populate_basepages'

Cc: Logan Gunthorpe <logang@deltatee.com>
Fixes: d95f1a542c ("RISC-V: Implement sparsemem")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/mm/init.c

@@ -458,7 +458,7 @@ void __init paging_init(void)
 	zone_sizes_init();
 }
 
-#ifdef CONFIG_SPARSEMEM
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
 int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 			       struct vmem_altmap *altmap)
 {