anonfd: fix missing declaration

The forward declaration of struct file_operations is
added to avoid compilation warnings.
Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/anon_inodes.h

@@ -8,6 +8,8 @@
 #ifndef _LINUX_ANON_INODES_H
 #define _LINUX_ANON_INODES_H
 
+struct file_operations;
+
 struct file *anon_inode_getfile(const char *name,
 				const struct file_operations *fops,
 				void *priv, int flags);