Add Q_GETNEXTQUOTA from Linux 4.6 to sys/quota.h.

This patch adds Q_GETNEXTQUOTA, new in Linux 4.6, to
sysdeps/unix/sysv/linux/sys/quota.h.

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).

	* sysdeps/unix/sysv/linux/sys/quota.h [_LINUX_QUOTA_VERSION >= 2]
	(Q_GETNEXTQUOTA): New macro.