fix(mac): Make constructor explicit

Author: lucasssvaz

cores/esp32/MacAddress.h

@@ -54,7 +54,7 @@ class MacAddress : public Printable {
   MacAddress(const uint8_t *macbytearray) : MacAddress(MAC6, macbytearray) {}
 
   MacAddress(const char *macstr);
-  MacAddress(const String &macstr);
+  explicit MacAddress(const String &macstr);
 
   virtual ~MacAddress() {}