[lightning-ln882h] Fix -Wregister warnings from CmBacktrace cmb_def.h (#376)

The LN882H SDK's CmBacktrace library (cmb_def.h) uses the 'register'
storage class specifier in inline assembly helpers (cmb_get_msp/psp/sp).
This keyword was removed in C++17 (P0001R1) and produces -Wregister
warnings when compiling with -std=c++17.

Add a fixup header that wraps the original cmb_def.h with GCC diagnostic
pragmas to suppress the warning without modifying SDK sources.

Author: LN882H Fix

cores/lightning-ln882h/base/sdk_private.h

@@ -48,7 +48,15 @@ extern "C" {
 // SDK
 #include "ln_kv_api.h"
 #include "ln_nvds.h"
+// cmb_def.h uses 'register' (removed in C++17); suppress -Wregister in C++ only
+#ifdef __cplusplus
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wregister"
+#endif
 #include "utils/debug/CmBacktrace/cm_backtrace.h"
+#ifdef __cplusplus
+#pragma GCC diagnostic pop
+#endif
 #include "utils/ln_psk_calc.h"
 #include "utils/power_mgmt/ln_pm.h"
 #include "utils/reboot_trace/reboot_trace.h"