build-sys: fix libvhost-user.a build

And actually link to it from vhost-user-bridge.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
diff --git a/Makefile b/Makefile
index cee6e28..64d3a9e 100644
--- a/Makefile
+++ b/Makefile
@@ -357,6 +357,7 @@
 # Build libraries
 
 libqemuutil.a: $(util-obj-y) $(trace-obj-y) $(stub-obj-y)
+libvhost-user.a: $(libvhost-user-obj-y)
 
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