Fix build on Windows/non-Unix

We'll need libcosmic to work on Windows and macOS if we want to use it
in the Keyboard Configurator.

`freedesktop-icons` doesn't seem to build on Windows, so only use it
on Unix systems. It does seem to build on macOS, so leave it as a
dependency there. A good solution for icon bundling on macOS/Windows will
need to be figured out at some point.

Cargo.toml

@@ -18,8 +18,8 @@ winit = ["iced/winit", "iced_winit"]
 applet = ["cosmic-panel-config", "sctk", "wayland"]
 winit_softbuffer = ["winit", "softbuffer"]
 winit_wgpu = ["winit", "wgpu"]
+
 [dependencies]
-freedesktop-icons = "0.2.2"
 apply = "0.3.0"
 derive_setters = "0.1.5"
 lazy_static = "1.4.0"
@@ -30,6 +30,9 @@ sctk = { package = "smithay-client-toolkit", git = "https://github.com/Smithay/c
 slotmap = "1.0.6"
 fraction = "0.13.0"
 
+[target.'cfg(unix)'.dependencies]
+freedesktop-icons = "0.2.2"
+
 [dependencies.cosmic-theme]
 git = "https://github.com/pop-os/cosmic-theme.git"