zmqpp: fix license information

Commit 85d28790 bumped the zmqpp Git
snapshot to 30d72d95f2cfdf9c5cedfd56747f549d65e65847.

However, the zmqpp licensing has changed (from MIT to
"LGPLv3+ with exceptions"), and the license file names have changed.

Fixes autobuild failures such as the following.

  http://autobuild.buildroot.net/results/a60/a6029fcb670cd27687a68cb96f0f14f5c13caf47



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>