contrib/rdmacm-mux: fix clang compilation

contrib/rdmacm-mux: fix clang compilation

Fix Commit a5d2f6f877 (contrib/rdmacm-mux: Add implementation
                       of RDMA User MAD multiplexer).

The above commit introduces a new contrib target, adding a global dependency
to libumad library in case pvrdma configuration option is enabled.
Clang forbids it:
    clang-6.0: error: -libumad: 'linker' input unused
              [-Werror,-Wunused-command-line-argument]

Fix by limiting the scope to the rdmacm-mux target itself.

Reported-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Tested-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190118124614.24548-4-marcel.apfelbaum@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>