python-treq: add missing BR2_INSTALL_LIBSTDCPP dependency

python-treq: add missing BR2_INSTALL_LIBSTDCPP dependency

python-treq selects python-pyopenssl, which depends on C++ support, but
this dependency was not propagated to python-treq, causing the following
kconfig warning:

warning: (BR2_PACKAGE_PYTHON_TREQ) selects BR2_PACKAGE_PYTHON_PYOPENSSL which has unmet direct dependencies ((BR2_PACKAGE_PYTHON || BR2_PACKAGE_PYTHON3) && BR2_INSTALL_LIBSTDCPP)

This commit fixes that by adding the appropriate dependency.

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