make: use -pthread when linking with -lrt

This is required when linking statically.  It is also consistent with
the rules for other zdtm tests that link with -lrt.
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/Makefile

@@ -261,7 +261,7 @@ sigaltstack:		override LDLIBS += -pthread
 shm:			override CFLAGS += -DNEW_IPC_NS
 msgque:			override CFLAGS += -DNEW_IPC_NS
 sem:			override CFLAGS += -DNEW_IPC_NS
-posix_timers:		override LDLIBS += -lrt
+posix_timers:		override LDLIBS += -lrt -pthread
 socket-tcp6: override CFLAGS += -D ZDTM_IPV6
 socket-tcpbuf6: override CFLAGS += -D ZDTM_IPV6
 socket-tcpbuf-local: override CFLAGS += -D ZDTM_TCP_LOCAL