Enable warnings when compiling the web extension

By accident, we don't use any compiler warnings when compiling the web
extension. This is bad.

We must either add -Wno-missing-prototypes or else declare a useless
prototype for webkit_web_extension_initialize_with_user_data().

https://bugzilla.gnome.org/show_bug.cgi?id=735491
ephy-web-extension: silence some warnings

https://bugzilla.gnome.org/show_bug.cgi?id=735491