Move INADDR_NONE definition to cpp

Fixes arduino/Arduino#1007

Author: facchinm

Diff for: api/IPAddress.cpp

@@ -114,3 +114,4 @@ size_t IPAddress::printTo(Print& p) const
     return n;
 }
 
+const IPAddress INADDR_NONE(0,0,0,0);

Diff for: api/IPAddress.h

@@ -72,5 +72,5 @@ class IPAddress : public Printable {
     friend class DNSClient;
 };
 
-const IPAddress INADDR_NONE(0,0,0,0);
+extern const IPAddress INADDR_NONE;