memblock tests: fix undefined reference to `early_pfn_to_nid'

commit 6a9531c ("memblock: fix crash when reserved memory is not
added to memory") introduce the usage of early_pfn_to_nid, which is not
defined in memblock tests.

The original definition of early_pfn_to_nid is defined in mm.h, so let
add this in the corresponding mm.h.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Yajun Deng <yajun.deng@linux.dev>
CC: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/20240402132701.29744-2-richard.weiyang@gmail.com
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>

tools/include/linux/mm.h

@@ -37,4 +37,9 @@ static inline void totalram_pages_add(long count)
 {
 }
 
+static inline int early_pfn_to_nid(unsigned long pfn)
+{
+	return 0;
+}
+
 #endif