python-service-identity: add missing BR2_INSTALL_LIBSTDCPP dependency

python-service-identity selects python-pyopenssl and python-pyasn, both
of which depend on C++ support, but this dependency was not propagated
to python-service-identity causing the following kconfig warning:

warning: (BR2_PACKAGE_PYTHON_SERVICE_IDENTITY) selects BR2_PACKAGE_PYTHON_PYOPENSSL which has unmet direct dependencies ((BR2_PACKAGE_PYTHON || BR2_PACKAGE_PYTHON3) && BR2_INSTALL_LIBSTDCPP)
warning: (BR2_PACKAGE_PYTHON_SERVICE_IDENTITY) selects BR2_PACKAGE_PYTHON_PYASN which has unmet direct dependencies ((BR2_PACKAGE_PYTHON || BR2_PACKAGE_PYTHON3) && BR2_INSTALL_LIBSTDCPP)
warning: (BR2_PACKAGE_PYTHON_SERVICE_IDENTITY) selects BR2_PACKAGE_PYTHON_PYASN_MODULES which has unmet direct dependencies ((BR2_PACKAGE_PYTHON || BR2_PACKAGE_PYTHON3) && BR2_INSTALL_LIBSTDCPP)

Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas: add Config.in comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>