Btrfs: extent map selftest: add missing void parameter to btrfs_test_extent_map

Add a missing void parameter to function btrfs_test_extent_map, fixes
sparse warning:

warning: non-ANSI function declaration of function 'btrfs_test_extent_map'
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tests/extent-map-tests.c

@@ -343,7 +343,7 @@ static void test_case_4(struct extent_map_tree *em_tree)
 	__test_case_4(em_tree, SZ_4K);
 }
 
-int btrfs_test_extent_map()
+int btrfs_test_extent_map(void)
 {
 	struct extent_map_tree *em_tree;