cgroup: add seq_file forward declaration for struct cftype

Recent header file changes for cgroup caused lots of warnings
about a missing struct seq_file form declaration for every
inclusion of include/linux/cgroup-defs.h.

As some files are built with -Werror, this leads to build
failure like:

                 from /git/arm-soc/drivers/gpu/drm/tilcdc/tilcdc_crtc.c:18:
/git/arm-soc/include/linux/cgroup-defs.h:354:25: error: 'struct seq_file' declared inside parameter list [-Werror]
cc1: all warnings being treated as errors
make[6]: *** [drivers/gpu/drm/tilcdc/tilcdc_crtc.o] Error 1

This patch adds the declaration, which resolves both the
warnings and the drm failure.

tj: Moved it where other type declarations are.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b4a04ab7 ("cgroup: separate out include/linux/cgroup-defs.h")
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -26,6 +26,7 @@ struct cgroup_taskset;
 struct kernfs_node;
 struct kernfs_ops;
 struct kernfs_open_file;
+struct seq_file;
 
 #define MAX_CGROUP_TYPE_NAMELEN 32
 #define MAX_CGROUP_ROOT_NAMELEN 64