fix zstd.h include for bundled zstd, fixes #6369

src/borg/algorithms/zstd-libselect.h

@@ -1,5 +1,5 @@
 #ifdef BORG_USE_BUNDLED_ZSTD
-#include <zstd.h>
+#include "zstd/lib/zstd.h"
 #else
 #include <zstd.h>
 #endif