Remove the `TARGET_OS_IPHONE` ifdef to include posix spawn headers for
iOS build. Previously https://github.com/libuv/libuv/pull/3257
introduced posix spawn with \_\_APPLE\_\_ platform only, which resulted
in a number of spawn related definitions not found for iOS (such as
`uv__posix_spawn_fncs_tag`).
Co-authored-by: Jameson Nash <vtjnash@gmail.com>