Fix "multiple definition of `debugfs_create_size_t'"

Introduced by 8adb711f3668b034e7b956fac951ed08b53e0d55 ("debugfs:
introduce stub for debugfs_create_size_t() when DEBUG_FS=n") and due to
a simple missing "static inline".

Reported-and-tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Acked-by: Greg KH <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/debugfs.h

@@ -162,7 +162,7 @@ static inline struct dentry *debugfs_create_x32(const char *name, mode_t mode,
 	return ERR_PTR(-ENODEV);
 }
 
-struct dentry *debugfs_create_size_t(const char *name, mode_t mode,
+static inline struct dentry *debugfs_create_size_t(const char *name, mode_t mode,
 				     struct dentry *parent,
 				     size_t *value)
 {