Private libraries and WrapOpenSSL were linked conditional to
QT_FEATURE_private_tests in the CMake file.
qsslkey_p.h and open ssl symbols were included conditional to developer
build. A developer build fails, with test enabled and private tests
disabled.
=> Change the CMake condition to QT_FEATURE_developer_build, to resolve
mismatch between CMake and cpp file.
Pick-to: 6.6 6.5
Change-Id: I3ac93b02701e467a0b548c35d441d35a45c4568b
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>