Update syscall-names.list for Linux 4.18.

This patch updates sysdeps/unix/sysv/linux/syscall-names.list for
Linux 4.18.  The io_pgetevents and rseq syscalls are added to the
kernel on various architectures, so need to be mentioned in this file.

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/syscall-names.list: Update kernel
	version to 4.18.
	(io_pgetevents): New syscall.
	(rseq): Likewise.