libxl: mark parameters in stub functions as unused

libxl: mark parameters in stub functions as unused

Hopefully this can fix issues like:

In file included from ../../src/libxl/xen_xl.c:24:0:
/home/osstest/build.147035.build-amd64-libvirt/xendist/usr/local/include/libxl.h: In function 'libxl_cpuid_apply_policy':
/home/osstest/build.147035.build-amd64-libvirt/xendist/usr/local/include/libxl.h:2345:56: error: unused parameter 'ctx' [-Werror=unused-parameter]
 static inline void libxl_cpuid_apply_policy(libxl_ctx *ctx, uint32_t domid) {}

Fixes: dacb80f9 ("tools/libxl: Remove libxl_cpuid_{set,apply_policy}() from the API")
Signed-off-by: Wei Liu <wl@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>