erofs: fix compile warnings when moving out include/trace/events/erofs.h

As Stephon reported [1], many compile warnings are raised when
moving out include/trace/events/erofs.h:

In file included from include/trace/events/erofs.h:8,
                 from <command-line>:
include/trace/events/erofs.h:28:37: warning: 'struct dentry' declared inside parameter list will not be visible outside of this definition or declaration
  TP_PROTO(struct inode *dir, struct dentry *dentry, unsigned int flags),
                                     ^~~~~~
include/linux/tracepoint.h:233:34: note: in definition of macro '__DECLARE_TRACE'
  static inline void trace_##name(proto)    \
                                  ^~~~~
include/linux/tracepoint.h:396:24: note: in expansion of macro 'PARAMS'
  __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args),  \
                        ^~~~~~
include/linux/tracepoint.h:532:2: note: in expansion of macro 'DECLARE_TRACE'
  DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
  ^~~~~~~~~~~~~
include/linux/tracepoint.h:532:22: note: in expansion of macro 'PARAMS'
  DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
                      ^~~~~~
include/trace/events/erofs.h:26:1: note: in expansion of macro 'TRACE_EVENT'
 TRACE_EVENT(erofs_lookup,
 ^~~~~~~~~~~
include/trace/events/erofs.h:28:2: note: in expansion of macro 'TP_PROTO'
  TP_PROTO(struct inode *dir, struct dentry *dentry, unsigned int flags),
  ^~~~~~~~

That makes me very confused since most original EROFS tracepoint code
was taken from f2fs, and finally I found

commit 43c78d8803 ("kbuild: compile-test kernel headers to ensure they are self-contained")

It seems these warnings are generated from KERNEL_HEADER_TEST feature and
ext4/f2fs tracepoint files were in blacklist.

Anyway, let's fix these issues for KERNEL_HEADER_TEST feature instead
of adding to blacklist...

[1] https://lore.kernel.org/lkml/20190826162432.11100665@canb.auug.org.au/

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190826132653.100731-1-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/trace/events/erofs.h

@@ -6,6 +6,9 @@
 #define _TRACE_EROFS_H
 
 #include <linux/tracepoint.h>
+#include <linux/fs.h>
+
+struct erofs_map_blocks;
 
 #define show_dev(dev)		MAJOR(dev), MINOR(dev)
 #define show_dev_nid(entry)	show_dev(entry->dev), entry->nid