f2fs: declare static function for __build_free_nids

This patch avoids build warning.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1805,7 +1805,7 @@ static void scan_nat_page(struct f2fs_sb_info *sbi,
 	}
 }
 
-void __build_free_nids(struct f2fs_sb_info *sbi, bool sync)
+static void __build_free_nids(struct f2fs_sb_info *sbi, bool sync)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);