node: remove deprecated register hint for C++17 compatibility

when building with `ZT_DEBUG=1` this hint produces a warning:

> node/Packet.cpp:335:43: error: 'register' storage class specifier is deprecated and incompatible with C++17 [-Werror,-Wdeprecated-register]
See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4340

node/Packet.cpp

@@ -332,7 +332,7 @@ static const int LZ4_minLength = (MFLIMIT+1);
 
 #define LZ4_STATIC_ASSERT(c)    { enum { LZ4_static_assert = 1/(int)(!!(c)) }; }   /* use only *after* variable declarations */
 
-static inline unsigned LZ4_NbCommonBytes (register reg_t val)
+static inline unsigned LZ4_NbCommonBytes (reg_t val)
 {
 	if (LZ4_isLittleEndian()) {
 	    if (sizeof(val)==8) {