Fix condition of the function to be targeted.

[hadashiA]

refs: #58, #60 (comment)

Besides using no_mangle, it seems that the #[export_name] attribute can emit symbol names without mangling.
Therefore, we would like to support this method of specification.
And only with #[no_mangle] or #[export_name = ...] are included. Otherwise, it print a message.

e.g)

#[export_name = "foo_bar"]
pub extern "C" fn foo()  {
    // ...
}

[DllImport(__DllName, EntryPoint = "foo_bar", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
public static extern int foo();