once NetBSD was presented, return its version

* defined by __NetBSD_Version__ on <sys/param.h> too.

src/lib/osinfo/libc.c

@@ -57,6 +57,9 @@ tor_libc_get_version_str(void)
 #ifdef __FreeBSD__
   return STR(__FreeBSD_version);
 #endif
+#ifdef __NetBSD_Version__
+  return STR(__NetBSD_Version__);
+#endif
 #ifdef OpenBSD
   return STR(OpenBSD);
 #endif /* defined(__BSD_VISIBLE) */