Force SCTK version to prevent pointer leaking

SCTK versions before 15.4 were leaking pointers when window got closed.
So the more windows you've got the pointers you'll keep around.

Fixes rust-windowing#2248.

Cargo.toml

@@ -89,7 +89,7 @@ features = [
 [target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "openbsd", target_os = "netbsd"))'.dependencies]
 wayland-client = { version = "0.29", default_features = false,  features = ["use_system_lib"], optional = true }
 wayland-protocols = { version = "0.29", features = [ "staging_protocols"], optional = true }
-sctk = { package = "smithay-client-toolkit", version = "0.15.1", default_features = false, features = ["calloop"],  optional = true }
+sctk = { package = "smithay-client-toolkit", version = "0.15.4", default_features = false, features = ["calloop"],  optional = true }
 mio = { version = "0.8", features = ["os-ext"], optional = true }
 x11-dl = { version = "2.18.5", optional = true }
 percent-encoding = { version = "2.0", optional = true }