dhcpcd: fix legal info

The latest version bump to 7.0.3 moved dhcpcd.c to under src/. Since the
tarball now has a LICENSE file, use that instead.

While at it, add a hash for that file.

Fixes:
http://autobuild.buildroot.net/results/f57/f5790e0a82a3b814d828d18eb71a3f6c422bb2b8/
http://autobuild.buildroot.net/results/6b9/6b930827913f9b4d2602d5777f4243ba7b295ca7/
http://autobuild.buildroot.net/results/603/603eda004a502f613e9b92cea4fb8cc9e4068255/



Cc: John Stile <johns@msli.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>