Add missing include in embed.cc

This changes fixes "error C2039: 'getline': is not a member of 'std'"
compilation error.

src/google/protobuf/compiler/js/embed.cc

@@ -32,6 +32,7 @@
 #include <cstdlib>
 #include <fstream>
 #include <iostream>
+#include <string>
 
 const char output_file[] = "well_known_types_embed.cc";