package/python-pysftp: add missing C++ dependency from paramiko

package/python-pysftp: add missing C++ dependency from paramiko

python-paramiko has a dependency on C++ support, which was added in
commit 2d7b73cf75daedd4a198a75546308fedae7c9467 in 2016.

When python-pysftp was added in commit
3b920487bad119abf5bfb10a237cedd3b5278c10 in 2020, this C++ dependency
was not propagated, even though python-pysftp selects python-paramiko.

This commit fixes this issue by propagating the dependency, which
fixes this warning:

WARNING: unmet direct dependencies detected for BR2_PACKAGE_PYTHON_PARAMIKO
  Depends on [n]: (BR2_PACKAGE_PYTHON [=n] || BR2_PACKAGE_PYTHON3 [=y]) && BR2_PACKAGE_PYTHON3 [=y] && BR2_INSTALL_LIBSTDCPP [=n]
  Selected by [y]:
  - BR2_PACKAGE_PYTHON_PYSFTP [=y] && (BR2_PACKAGE_PYTHON [=n] || BR2_PACKAGE_PYTHON3 [=y]) && BR2_PACKAGE_PYTHON3 [=y]

That occurs with configuration with C++ disabled, but python-pysftp
enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>