build: remove spdk.so library from .PHONY

Targets listed in .PHONY are always built - but
in this case if the underlying spdk.so.$VER library
hasn't been modified, there's no reason to redo the
symlink.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ife60f1de6dd6d0997bc5adf40d039cfd1dbc88ec
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4463


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>