tests: fix include in input-timestamps-helper.c

No need to use the protocol directory prefix. This may even be necessary
for the Meson build.

Signed-off-by: Daniel Stone <daniels@collabora.com>

Extracted from the patch adding the Meson build system.

Signed-off-by: Pekka Paalanen <pq@iki.fi>

tests/input-timestamps-helper.c

@@ -31,7 +31,7 @@
 #include <time.h>
 
 #include "input-timestamps-helper.h"
-#include "protocol/input-timestamps-unstable-v1-client-protocol.h"
+#include "input-timestamps-unstable-v1-client-protocol.h"
 #include "shared/timespec-util.h"
 #include "shared/zalloc.h"
 #include "weston-test-client-helper.h"