Add <optional> include to proxy_asm_api.h (proxy-wasm#160)

proxy_asm_api.h references `std::optional`—see, for example, `getProperty`—but was missing the include for it.

Signed-off-by: Benjamin Peterson <benjamin@engflow.com>
Co-authored-by: Michael Warres <mpw@google.com>

Author: benjaminp

proxy_wasm_api.h

@@ -28,6 +28,7 @@
 #include <cstring>
 #include <functional>
 #include <memory>
+#include <optional>
 #include <string>
 #include <tuple>
 #include <unordered_map>