Fix musl build by letting Rust infer the correct type for SIOCGIFMTU

[vkrasnov]

Fixes #47

[Lekensteyn]

I guess that this is similar to casting a pointer to void* in C?

Do you happen to have a reference for how this works? I can't find the _ case here:
https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions

[vkrasnov]

I guess that this is similar to casting a pointer to void* in C?

Only where pointer is expected. In this case it just coerces the integer to the correct bitwidth (something that C does automatically)

https://doc.rust-lang.org/reference/type-coercions.html
https://doc.rust-lang.org/reference/expressions/operator-expr.html#type-cast-expressions

[Lekensteyn]

Ok, so apparently this is the "inferred type" which is documented here:
https://doc.rust-lang.org/reference/types/inferred.html

The change looks good to me, but perhaps you could clarify the commit message?

[ashleygwilliams]

yay i can make the binaries for the musl now! thank u :)