repeat_key.c: add implementation for get_last_record (#26263)

The `get_last_record` signature was present in repeat_key.h but without
any implementation in repeat_key.c which caused compilation errors for
any user of `get_last_record`.

quantum/repeat_key.c

@@ -45,6 +45,10 @@ uint16_t get_last_keycode(void) {
     return last_record.keycode;
 }
 
+const keyrecord_t* get_last_record(void) {
+    return &last_record;
+}
+
 uint8_t get_last_mods(void) {
     return last_mods;
 }