package/dhcpdump: fix build without pod2man

package/dhcpdump: fix build without pod2man

Use dhcpdump target to avoid building dhcpdump.8 as it will raise the
following build failure if pod2man is not available:

pod2man --section 8 \
	--date "23 June 2008" \
	--name "DHCPDUMP" \
	--center "User Contributed Software" \
	dhcpdump.pod dhcpdump.8
/bin/sh: pod2man: command not found
make[1]: *** [Makefile:11: dhcpdump.8] Error 127

Fixes:
 - http://autobuild.buildroot.org/results/db3be149ec71de8376f685a6a9f027191d9bccc9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>