netdissect-stdinc: get rid of an extra definition of strdup.

We first unconditionally define it, and then later only define it if
it's already defined.  Microsoft defines it for us, but has deprecated
that definition because it pollutes the ISO C namespace with a POSIXism;
try just doing the conditional definition.

Author: guyharris

netdissect-stdinc.h

@@ -166,7 +166,6 @@
    */
   #define isatty _isatty
   #define stat _stat
-  #define strdup _strdup
   #define open _open
   #define read _read
   #define close _close