[rtdbg] Fix compile error when using LOG_HEX(...) function but RT_USING_ULOG not defined

include/rtdbg.h

@@ -178,6 +178,8 @@ extern "C" {
 
 #define LOG_RAW(...)         dbg_raw(__VA_ARGS__)
 
+#define LOG_HEX(name, width, buf, size)
+
 #endif /* defined(RT_USING_ULOG) && define(DBG_ENABLE) */
 
 #ifdef __cplusplus