shared_lib: add libspdk_thread to libspdk.so

This was missed when splitting thread into its own library; the thread
symbols were missing from libspdk.so.

Change-Id: Ibcd1ea8cc5be1340cc4b848c81b4c92b036675a8
Fixes: 02d75f62 ("thread: Move threading abstraction code out of util")
Fixes: 6f46e272 ("build: add combined shared library (libspdk.so)")
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/417171


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>