Include <numeric>

Fixes Linux build failures.

2016/puzzle-20-01.cc

@@ -1,4 +1,5 @@
 #include <iostream>
+#include <numeric>
 #include <regex>
 #include <string>
 
@@ -84,4 +85,4 @@ auto main() -> int
 
   std::cout << "First available IP address: " << ranges.inverse().lowest() << '\n';
   return 0;
-}
+}