zmqpp: fix compilation with some toolchains

Fixes:
http://autobuild.buildroot.net/results/d9ec39dc5829aac1b25bfea7484458fcebd68412/

add cstdlib header to ensure EXIT_FAILURE presence

Using some cross-compiling toolchains, EXIT_FAILURE is not always
properly defined without this inclusion.

Patch is upstream:
https://github.com/zeromq/zmqpp/pull/109



[Thomas: use a patch formatted with git format-patch, rather than the
output of 'git show'.]

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>