iproute2: avoid errors from double-installing manpages

iproute2: avoid errors from double-installing manpages

Three manpages in man8 are listed twice in MAN8PAGES (both directly and
in TARGETS) which causes the install command to spit our a couple of
warnings as below and exiting with non-zero exit code....

make[3]: Entering directory `/tmp/buildd/iproute-20121001/man/man8'
install -m 0755 -d /tmp/buildd/iproute-20121001/debian/tmp/usr/share/man/man8
install -m 0644 ip-address.8 ip-link.8 ip-route.8 ip.8 arpd.8 lnstat.8 routel.8 rtacct.8 rtmon.8 ss.8 tc.8 tc-bfifo.8 tc-cbq.8 tc-cbq-details.8 tc-choke.8 tc-codel.8 tc-drr.8 tc-ematch.8 tc-fq_codel.8 tc-hfsc.8 tc-htb.8 tc-netem.8 tc-pfifo.8 tc-pfifo_fast.8 tc-prio.8 tc-red.8 tc-sfb.8 tc-sfq.8 tc-stab.8 tc-tbf.8 bridge.8 rtstat.8 ctstat.8 nstat.8 routef.8 ip-address.8 ip-addrlabel.8 ip-l2tp.8 ip-link.8 ip-maddress.8 ip-monitor.8 ip-mroute.8 ip-neighbour.8 ip-netns.8 ip-ntable.8 ip-route.8 ip-rule.8 ip-tunnel.8 ip-xfrm.8 /tmp/buildd/iproute-20121001/debian/tmp/usr/share/man/man8
install: will not overwrite just-created `/tmp/buildd/iproute-20121001/debian/tmp/usr/share/man/man8/ip-address.8' with `ip-address.8'
install: will not overwrite just-created `/tmp/buildd/iproute-20121001/debian/tmp/usr/share/man/man8/ip-link.8' with `ip-link.8'
install: will not overwrite just-created `/tmp/buildd/iproute-20121001/debian/tmp/usr/share/man/man8/ip-route.8' with `ip-route.8'
make[3]: *** [install] Error 1
make[3]: Leaving directory `/tmp/buildd/iproute-20121001/man/man8'
make[2]: *** [install] Error 2
make[2]: Leaving directory `/tmp/buildd/iproute-20121001/man'

Signed-off-by: Andreas Henriksson <andreas@fatal.se>