Fix build failures with OPT = 0 due to inline functions (#19767)

Co-authored-by: Joel Challis <[email protected]>

platforms/synchronization_util.c

@@ -0,0 +1,17 @@
+// Copyright 2023 Sergey Vlasov (@sigprof)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "synchronization_util.h"
+
+// Generate out-of-line copies for inline functions defined in synchronization_util.h.
+
+#if !defined(PLATFORM_SUPPORTS_SYNCHRONIZATION)
+#    if defined(SPLIT_KEYBOARD)
+extern inline void split_shared_memory_lock(void);
+extern inline void split_shared_memory_unlock(void);
+#    endif
+#endif
+
+#if defined(SPLIT_KEYBOARD)
+QMK_IMPLEMENT_AUTOUNLOCK_HELPERS(split_shared_memory)
+#endif