HID: ishtp-hid-client: Move variable to where it's actually used

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/hid/intel-ish-hid/ishtp-hid.c:11:
     drivers/hid/intel-ish-hid/ishtp-hid.h:24:21: warning: ‘hid_ishtp_guid’ defined but not used [-Wunused-const-variable=]

Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: Daniel Drubin <daniel.drubin@intel.com>
Cc: linux-input@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>