package/dhcp: fix autoreconf for libtool-enabled build

Upstream provides two configure.ac versions, the default one does not
use libtool. For libtool-enabled builds the default configure.ac calls
https://gitlab.isc.org/isc-projects/dhcp/blob/master/configure.ac#L873
https://gitlab.isc.org/isc-projects/dhcp/blob/master/config+lt

which copies configure.ac+lt to configure.ac and starts autoreconf.

On host systems lacking autoreconf this causes a build error:

configure: WARNING: legacy configure is used but libtool is enabled. Trying to recover...
/home/buildroot/autobuild/instance-2/output-1/build/dhcp-4.4.1/config+lt: 20:
 /home/buildroot/autobuild/instance-2/output-1/build/dhcp-4.4.1/config+lt: autoreconf: not found
autoreconf failed

Fixes:
http://autobuild.buildroot.net/results/16f/16f800dd844572336c2fbba3d123604405382a84/
http://autobuild.buildroot.net/results/eac/eac1e7b25128476eccf40aece612e13a1e0637dc/
http://autobuild.buildroot.net/results/8fc/8fcfe7b40af3ae42549351c70466b4dd388820e3/
http://autobuild.buildroot.net/results/7ff/7ff82708b9dea09f1447f0ef94b9bb21fca73196/
http://autobuild.buildroot.net/results/1c5/1c5f401ce7eef13f5ca4603ca8d7c67da249d7c9/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>