Exporting the -static CMake targets when both shared and static libraries built.

[fpagliughi]

When just the static library is built, the CMake targets are properly exported (i.e. xxx-static, like eclipse-paho-mqtt-c::paho-mqtt3a-static, etc).

But when both the shared and static libraries are built, only the shared targets are exported. The static ones are not - but should be.

This fixes the issue so that the -static CMake targets are always exported when the static libraries are built.

This problem is causing broken builds for the Paho C++ library. See: https://github.com/eclipse/paho.mqtt.cpp/issues/500