Fix an error when including debug.h from C

Author: igrr

cores/esp8266/debug.h

@@ -5,7 +5,10 @@
 // #define DEBUGV(...) ets_printf(__VA_ARGS__)
 #define DEBUGV(...)
 
+#ifdef __cplusplus
 void hexdump(uint8_t *mem, uint32_t len, uint8_t cols = 16);
-
+#else
+void hexdump(uint8_t *mem, uint32_t len, uint8_t cols);
+#endif
 
 #endif//ARD_DEBUG_H