contrib/rdmacm-mux: fix clang compilation

Fix Commit a5d2f6f8 (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: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NYuval Shaia <yuval.shaia@oracle.com>
Tested-by: NCornelia Huck <cohuck@redhat.com>
Message-Id: <20190118124614.24548-4-marcel.apfelbaum@gmail.com>
Signed-off-by: NMarcel Apfelbaum <marcel.apfelbaum@gmail.com>

Makefile

@@ -581,6 +581,8 @@ vhost-user-scsi$(EXESUF): $(vhost-user-scsi-obj-y) libvhost-user.a
 	$(call LINK, $^)
 vhost-user-blk$(EXESUF): $(vhost-user-blk-obj-y) libvhost-user.a
 	$(call LINK, $^)
+
+rdmacm-mux$(EXESUF): LIBS += "-libumad"
 rdmacm-mux$(EXESUF): $(rdmacm-mux-obj-y) $(COMMON_LDADDS)
 	$(call LINK, $^)
 

contrib/rdmacm-mux/Makefile.objs

 ifdef CONFIG_PVRDMA
-CFLAGS += -libumad
 rdmacm-mux-obj-y = main.o
 endif