[clang] Fix missing header after libc++ update

Upstream libc++ has changed a transitive header dependency. The upstream
PR is here: https://github.com/llvm/llvm-project/pull/80418 and no
longer transitively brought <cstdlib>.

Bug: 324466743
Change-Id: I7b438aec2da58a60c419052c0725f0233d7a1bdd
diff --git a/android-emu/aemu/base/Optional.h b/android-emu/aemu/base/Optional.h
index d0a8aa7..802a793 100644
--- a/android-emu/aemu/base/Optional.h
+++ b/android-emu/aemu/base/Optional.h
@@ -24,6 +24,7 @@
 #include <utility>
 
 #include <cstddef>
+#include <cstdlib>
 
 // Optional<T> - a template class to store an optional value of type T.
 //