Add operator != to IPAddress

Fixes arduino/Arduino#7336

Author: facchinm

Diff for: api/IPAddress.h

@@ -52,6 +52,7 @@ class IPAddress : public Printable {
     // to a four-byte uint8_t array is expected
     operator uint32_t() const { return _address.dword; };
     bool operator==(const IPAddress& addr) const { return _address.dword == addr._address.dword; };
+    bool operator!=(const IPAddress& addr) const { return _address.dword != addr._address.dword; };
     bool operator==(const uint8_t* addr) const;
 
     // Overloaded index operator to allow getting and setting individual octets of the address