[client] tests: include clipboard protocol constants

The X11 clipboard test directly checks the protocol representation chunk
limit. Include the header that defines the constant instead of relying on
a transitive include from the implementation translation unit.

client/tests/x11_clipboard_test.c

@@ -26,6 +26,7 @@
 #include "core/clipboard_files.h"
 
 #include "common/debug.h"
+#include "common/KVMFRClipboard.h"
 
 #include <stdint.h>
 #include <stdio.h>