Make zenity related deps optional (#141)

Cargo.toml

@@ -14,7 +14,7 @@ documentation = "https://docs.rs/rfd"
 default = ["gtk3"]
 file-handle-inner = []
 gtk3 = ["gtk-sys", "glib-sys", "gobject-sys"]
-xdg-portal = ["ashpd", "urlencoding", "pollster"]
+xdg-portal = ["ashpd", "urlencoding", "pollster", "async-io", "futures-util"]
 common-controls-v6 = ["windows-sys/Win32_UI_Controls"]
 
 [dev-dependencies]
@@ -42,8 +42,8 @@ windows-sys = { version = "0.48", features = [
 
 [target.'cfg(any(target_os = "linux", target_os = "freebsd", target_os = "dragonfly", target_os = "netbsd", target_os = "openbsd"))'.dependencies]
 # Make sure that this is in sync with zbus, to avoid duplicated deps
-async-io = "1.3"
-futures-util = { version = "0.3", default-features = false, features = ["io"] }
+async-io = { version = "1.3", optional = true }
+futures-util = { version = "0.3", optional = true, default-features = false, features = ["io"] }
 
 # XDG Desktop Portal
 ashpd = { version = "0.6", optional = true }