OpenBLAS

Fix exported OpenBLASTargets.cmake

When both BUILD_SHARED_LIBS and BUILD_STATIC_LIBS are enabled,
cmake export both of them to OpenBLASTargets under tha same name `OpenBLAS::OpenBLAS`
which leads to fatal error about OpenBLAS::OpenBLAS being both static and shared target.
This change makes cmake export only the shared library in that case.
There is another solution to treat them as components,
but I am afraid that will make it backward incompatible.