Add F_SEAL_FUTURE_WRITE from Linux 5.1 to bits/fcntl-linux.h.

This patch adds the new F_SEAL_FUTURE_WRITE constant from Linux 5.1 to
bits/fcntl-linux.h.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h [__USE_GNU]
	(F_SEAL_FUTURE_WRITE): New macro.