package/opentracing-cpp: needs dynamic library support

opentracing-cpp requires dlfcn.h from src/dynamic_load_unix.cpp.
This file is compiled unconditionally.

Disable opentracing-cpp on BR2_STATIC_LIBS configurations.

Fixes: http://autobuild.buildroot.net/results/454173aef9ff7c808294a974088d7682cad240a8/



Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>