AK: Include Assertions.h in StdLibExtras.h

Author: https://github.com/RealKC
Commit: 8f7aa1e03a
Pull-request: https://github.com/SerenityOS/serenity/pull/5688
Issue: https://github.com/SerenityOS/serenity/issues/4282
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/BenWiederhake
Reviewed-by: https://github.com/awesomekling

AK/StdLibExtras.h

@@ -26,6 +26,8 @@
 
 #pragma once
 
+#include <AK/Assertions.h>
+
 constexpr unsigned round_up_to_power_of_two(unsigned value, unsigned power_of_two)
 {
     return ((value - 1) & ~(power_of_two - 1)) + power_of_two;