hw/dmx/config: Link directly with libdmxconfig.a

When building xserver with slibtool (https://dev.midipix.org/cross/slibtool)
the build will fail.

  /usr/bin/ld: cannot find -ldmxconfig

This is because xserver creates libdmxconfig.a internally and then links with
the linker flag -ldmxconfig. However according to automake documentation the
-lfoo linker flags should only be used for external dependencies and all
internal libraries should be linked with the libtool archive file (.la) or
the static archive (.a) when the former is not available.

GNU libtool is far more permissive and happens to silently obscure this issue
while slibtool fails because it instead sees '-L./.libs -ldmxconfig'.
5 jobs for dmx in 3 minutes and 46 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-buster #11303957


  Build And Test
passed autotools #11303958


passed meson #11303960


passed meson-noglamor #11303962


passed mingw-cross-build #11303963