msgpack: disables tests

tests are enabled if gperf and zlib are found and they fail on:
/home/buildroot/autobuild/run/instance-0/output/build/msgpack-2.1.5/include/msgpack/v1/object.hpp:652:34:
error: 'void* memcpy(void*, const void*, size_t)' copying an object of non-trivial type 'struct msgpack::v2::object' from an array of 'const msgpack_object' {aka 'const struct msgpack_object'} [-Werror=class-memaccess]
     std::memcpy(&o, &v, sizeof(v));

So disable them.

Fixes:
 - http://autobuild.buildroot.org/results/7d7aa9723f02f9bc78dbf6248674be4d402199bf



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>