Fix make test:all failures seen on macOS Tahoe (#26136)

platforms/timer.c

@@ -1,7 +1,11 @@
 // Copyright 2023 Sergey Vlasov (@sigprof)
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-#include "timer.h"
+// Use an angle-bracket include so the header is located via the include
+// search path rather than source-relative lookup. timer.h uses
+// #include_next to chain to the active platform's _timer.h, and that
+// only has well-defined semantics when timer.h itself was found via -I.
+#include <timer.h>
 
 // Generate out-of-line copies for inline functions defined in timer.h.
 extern inline fast_timer_t timer_read_fast(void);