ntpd/ntp_sandbox.c: allow readv() for glibc nscd getaddrinfo() provider

Otherwise, ntpd crashes from time to time with the following stack trace:
#0  0x00007f5763bfac4d in readv () from /lib64/libc.so.6
#1  0x00007f5763c48b4c in __readvall () from /lib64/libc.so.6
#2  0x00007f5763c467ed in nscd_gethst_r () from /lib64/libc.so.6
#3  0x00007f5763c46c0d in __nscd_gethostbyname2_r () from /lib64/libc.so.6
#4  0x00007f5763c15a2a in gethostbyname2_r () from /lib64/libc.so.6
#5  0x00007f5763bed3ca in gaih_inet.constprop () from /lib64/libc.so.6
#6  0x00007f5763bee225 in getaddrinfo () from /lib64/libc.so.6
#7  0x000055fcf0ad6544 in open_TCP_socket ()
#8  0x000055fcf0ad79b4 in nts_probe ()
#9  0x000055fcf0ac37c4 in dns_lookup ()
#10 0x00007f5763b8205a in start_thread () from /lib64/libc.so.6
#11 0x00007f5763c05d1c in clone3 () from /lib64/libc.so.6

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>