Add missing const

The key part of the pair isn't mutable.

Change-Id: I8898b54131e73fba9caf6f3e3396662c51615027
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/aos_graph_channels.cc b/aos/aos_graph_channels.cc
index e96494a..7eb22ba 100644
--- a/aos/aos_graph_channels.cc
+++ b/aos/aos_graph_channels.cc
@@ -185,7 +185,7 @@
   // Now generate graphvis compatible output.
   std::stringstream graph_out;
   graph_out << "digraph g {" << std::endl;
-  for (const std::pair<const aos::Channel *, ChannelConnections> &c :
+  for (const std::pair<const aos::Channel *const, ChannelConnections> &c :
        connections) {
     const std::string channel = absl::StrCat(
         c.first->name()->string_view(), "\n", c.first->type()->string_view());