compilation error in std for target_os="ios"

[semarie]

This patch corrects the following errors:

../src/libstd/env.rs:569:42: 569:63 error: unresolved name `super::os::DLL_PREFIX`
../src/libstd/env.rs:569     pub const DLL_PREFIX: &'static str = super::os::DLL_PREFIX;
                                                                  ^~~~~~~~~~~~~~~~~~~~~
../src/libstd/env.rs:574:42: 574:63 error: unresolved name `super::os::DLL_SUFFIX`
../src/libstd/env.rs:574     pub const DLL_SUFFIX: &'static str = super::os::DLL_SUFFIX;
                                                                  ^~~~~~~~~~~~~~~~~~~~~
../src/libstd/env.rs:579:45: 579:69 error: unresolved name `super::os::DLL_EXTENSION`
../src/libstd/env.rs:579     pub const DLL_EXTENSION: &'static str = super::os::DLL_EXTENSION;
                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~

The patch is untested: only the compilation was done. The code follow "macos" definition.

[rust-highfive]

r? @huonw

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[alexcrichton]

Duplicate of #23029, but thanks!

[semarie]

my bad, I don't have checked for duplicate first.