x86/hyperv: change hv_tlb_flush_ex to fix clang build

x86/hyperv: change hv_tlb_flush_ex to fix clang build

Clang complains:

In file included from synic.c:15:
/builds/xen-project/xen/xen/include/asm/guest/hyperv-tlfs.h:900:18: error: field 'hv_vp_set' with variable sized type 'struct hv_vpset' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
        struct hv_vpset hv_vp_set;
                        ^
1 error generated.
/builds/xen-project/xen/xen/Rules.mk:198: recipe for target 'synic.o' failed
make[6]: *** [synic.o] Error 1

Comment out the last variable size array from hv_tlb_flush_ex to fix
clang builds.

Fixes: bbba482664 ("x86: import hyperv-tlfs.h from Linux")
Signed-off-by: Wei Liu <liuwe@microsoft.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>