nginx-module-opentracing: adjusted libstdc++-devel build requirement.

The libjaegertracing_plugin.so requires libstdc++.a, and that library
is provided by the corresponding package in Fedora and RHEL 8, while
older RHEL 7 does not have it out of the box.

Additionally, Jaeger plugin requires gcc >= 4.9 while RHEL 7 has 4.8,
thus making it effectively impossible to build the plugin without
installing additional software collections from Redhat (e.g. devtoolset-8).