Add missing include of "asio/error.hpp" header.

asio/include/asio/detail/impl/posix_event.ipp

diff --git a/asio/include/asio/detail/impl/posix_event.ipp b/asio/include/asio/detail/impl/posix_event.ipp
index b658d4b..73bac9a 100644
--- a/asio/include/asio/detail/impl/posix_event.ipp
+++ b/asio/include/asio/detail/impl/posix_event.ipp
@@ -21,6 +21,7 @@
 
 #include "asio/detail/posix_event.hpp"
 #include "asio/detail/throw_error.hpp"
+#include "asio/error.hpp"
 
 #include "asio/detail/push_options.hpp"