mosh: Fix compilation with libcxx

Partial backport of upstream patch.``

Signed-off-by: Rosen Penev <rosenp@gmail.com>

net/mosh/patches/020-std.patch

@@ -0,0 +1,10 @@
+--- a/src/network/network.cc
++++ b/src/network/network.cc
+@@ -60,7 +60,6 @@
+ #define AI_NUMERICSERV 0
+ #endif
+ 
+-using namespace std;
+ using namespace Network;
+ using namespace Crypto;
+