Fix support for vector<bool> serialization with libc++

The vector<bool> serialization supported added in
#86 commit
1c6fb04 was only tested with libstdc++ not
libc++ and caused a test compilation failure when compiling on macos, reported
by edilmedeiros in #92

Fix the issue that was causing the build to fail: trying to use using vector's
reference type instead of const_reference type when serializing the vector.

Fixes #92

Author: ryanofsky

include/mp/proxy-types.h

@@ -881,7 +881,7 @@ ::capnp::Void BuildPrimitive(InvokeContext& invoke_context, Value&&, TypeList<::
     return {};
 }
 
-inline static bool BuildPrimitive(InvokeContext& invoke_context, std::vector<bool>::reference value, TypeList<bool>)
+inline static bool BuildPrimitive(InvokeContext& invoke_context, std::vector<bool>::const_reference value, TypeList<bool>)
 {
     return value;
 }