cmake: hide symbols import from other libraries in libcls_*

cmake: hide symbols import from other libraries in libcls_*

so they will not be involved when resolving symbols. ld tries to
keep a shared library around even if it fails to load it if it offers
some unique symbols. in that case, the library will not be properly
unloaded, and even worse it will interfere with following dlopen()
calls, because it is marked with NODELETE by dlopen(). if it has some
unresolved symbol and does offer some "unique" symbols required by
the library to be loaded, the library will fail to load, despite the
fact that the "unique" symbol is also offered by the executable.

for more details, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60731 and
https://sourceware.org/bugzilla/show_bug.cgi?id=14577

Fixes: http://tracker.ceph.com/issues/23517
Signed-off-by: Kefu Chai <kchai@redhat.com>