tools/libxc: unused attribute in DECLARE_HYPERCALL_BUFFER_SHADOW

tools/libxc: unused attribute in DECLARE_HYPERCALL_BUFFER_SHADOW

There are cases where we only need to use the hypercall buffer data,
and do not use the xc_hypercall_buffer_t struct.
DECLARE_HYPERCALL_BUFFER_SHADOW defines a user pointer that can allow
us to access the hypercall buffer data but it also defines a
xc_hypercall_buffer_t that we don't use, the compiler will report arg
unused error.
Add __attribute__((unused)) before xc_hypercall_buffer_t to avoid
the compiler error.

Example cases:
In send_all_pages(), we only need to use the hypercall buffer data
which is a dirty bitmap, we set the dirty bitmap to all dirty and call
send_dirty_pages, we will not use the xc_hypercall_buffer_t and hypercall
to retrieve the dirty bitmap.
In send_some_pages(), we will also only need to use the dirty_bitmap.
the retrieve dirty bitmap hypercall are done by the caller.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>