shared_lib: add as_needed to the libspdk.so linker script

specifying as_needed in the linker script prevents us from creating more
runtime dependencies than necessary for a given application linking to
libspdk.so

Change-Id: Ifd3788e210c5e6a0e19749cb575c3dfb494a41d2
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/427808


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Lance Hartmann <lance.hartmann@oracle.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>