xenctrl-ext: link stubs explicitly agains libxenctrl (#7135)

The xenctrlext_stubs use libxenctrl symbols (e.g. xc_cpumap_alloc) but
did not declare it as a link dependency. So, the produced
dllxenctrl_ext_stubs.so had no NEEDED entry for libxenctrl.

When I try to run `dune utop` on my Debian Trixie the loading fails
with:
  Fatal error: cannot load shared library dllxenctrl_ext_stubs
  Reason: .../ocaml/libs/xenctrl-ext/dllxenctrl_ext_stubs.so:
          undefined symbol: xc_cpumap_alloc

This patch adds -lxenctrl to c_library_flags so the stubs declare the
dependency themselves.

Author: last-genius

ocaml/libs/xenctrl-ext/dune

@@ -6,5 +6,6 @@
   (language c)
   (names xenctrlext_stubs)
  )
+ (c_library_flags (-lxenctrl))
 )