test: fix compiler warnings for missing field initializers

litest-trackpoint.c:38:1: warning: missing initializer for field 'touch_down'
of 'struct litest_device_interface' [-Wmissing-field-initializers]

and similar

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

test/litest-mouse.c

@@ -35,6 +35,7 @@ static void litest_mouse_setup(void)
 }
 
 static struct litest_device_interface interface = {
+	NULL
 };
 
 static struct input_id input_id = {

test/litest-trackpoint.c

@@ -35,6 +35,7 @@ static void litest_trackpoint_setup(void)
 }
 
 static struct litest_device_interface interface = {
+	NULL
 };
 
 static struct input_id input_id = {