mk: add a placeholder map file.

There are a lot of SPDK libraries that don't
export any symbols and only have a conxtructor function
which registers function pointers. For these libraries,
we will want a placeholder map file that doesn't export
any symbols.

Signed-off-by: Seth Howell <seth.howell@intel.com>
Change-Id: Id84ba51235a77884164843fe3508d7a458835402
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/2311


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>