Change io_service to io_context

This was renamed a while back in boost to be compliant with the
std::executors proposal.

Change-Id: I67eb15038f2aa6957738fa5e3f6c14fd66460b7a
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/attn/attn_main.cpp b/attn/attn_main.cpp
index 0c3f250..84c3bba 100644
--- a/attn/attn_main.cpp
+++ b/attn/attn_main.cpp
@@ -12,7 +12,7 @@
 
     gpiod_line* line; // gpio line to monitor
 
-    boost::asio::io_service io; // async io monitoring service
+    boost::asio::io_context io; // async io monitoring service
 
     // GPIO line configuration (falling edge, active low)
     struct gpiod_line_request_config config