contrib/rdmacm-mux: convert to Meson

We can use config-host.mak to decide whether the tool has to be built,
apart from that the conversion is straightforward.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index 4c067d9..c60ec58 100644
--- a/meson.build
+++ b/meson.build
@@ -260,6 +260,8 @@
 # Other build targets
 
 if have_tools
+  subdir('contrib/rdmacm-mux')
+
   if 'CONFIG_VHOST_USER' in config_host
     subdir('contrib/libvhost-user')
     subdir('contrib/vhost-user-blk')