add missing unstable feature to core/lib.rs

Author: RalfJung

library/core/src/lib.rs

@@ -205,6 +205,7 @@
 #![feature(arm_target_feature)]
 #![feature(avx512_target_feature)]
 #![feature(hexagon_target_feature)]
+#![feature(keylocker_x86)]
 #![feature(loongarch_target_feature)]
 #![feature(mips_target_feature)]
 #![feature(powerpc_target_feature)]