debugfs: fix sparse warnings

extern does not belong in C files, move declaration to linux/debugfs.h
fs/debugfs/file.c:42:30: warning: symbol 'debugfs_file_operations' was not declared. Should it be static?
fs/debugfs/file.c:54:31: warning: symbol 'debugfs_link_operations' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>