Mark internal fcntl functions hidden

Since internal fcntl functions are only used internally in ld.so and
libc.so, they can be made hidden.

	[BZ #19122]
	* include/fcntl.h (__libc_fcntl): Add attribute_hidden.
	[IS_IN (rtld)] (__open): Likewise.
	[IS_IN (rtld)] (__fcntl): Likewise.