gnetworking.h.in: move "#undef interface"

The win32 headers do:

  #define interface struct

which is just evil and breaks other code that assumes it can use
"interface" as a variable name. gnetworking.h was supposed to be doing
"#undef interface" after including the win headers, but it did it too
soon, resulting in it getting redefined by a later include. Fix this.

https://bugzilla.gnome.org/show_bug.cgi?id=738551