Fix Windows API functions

[filmor]

When adding the newer functions, I messed up the order and missed one function, which breaks rustler-sys for all newer API versions on Windows. Other OS are not affected by this, as they link the symbols directly instead of getting passed a struct.

Making the rustler_sys functions extern "C" just ensures that they look the same, independent of the operation system.

[filmor]

@philss @josevalim This finally fixes the issues that we ran into with enif_term_type. The name was simply assigned the wrong function because when adding the newer API functions, I wasn't aware of the order-sensitivity :/

[josevalim]

@filmor hooray! How did you even find this one out? :D

[filmor]

While working on the dyncall support in #635, I got really strange errors on Windows when using enif_init_resource_type that referenced enif_term_type, and then I had a closer look how the API functions are referenced on Windows. :)