libnvdimm/e820: Fix build error without MEMORY_HOTPLUG

If MEMORY_HOTPLUG is n, build fails:

drivers/nvdimm/e820.c: In function ‘e820_register_one’:
drivers/nvdimm/e820.c:24:12: error: implicit declaration of function ‘phys_to_target_node’; did you mean ‘set_page_node’? [-Werror=implicit-function-declaration]
  int nid = phys_to_target_node(res->start);
            ^~~~~~~~~~~~~~~~~~~
            set_page_node

Fixes: 7b27a86 ("libnvdimm/e820: Retrieve and populate correct 'target_node' info")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>

drivers/nvdimm/e820.c

@@ -17,6 +17,13 @@ static int e820_pmem_remove(struct platform_device *pdev)
 	return 0;
 }
 
+#ifndef CONFIG_MEMORY_HOTPLUG
+static inline int phys_to_target_node(u64 start)
+{
+	return 0;
+}
+#endif
+
 static int e820_register_one(struct resource *res, void *data)
 {
 	struct nd_region_desc ndr_desc;