Fix C+1y warnings from Clang.

When compiling with Clang 3.4, I get these warnings about the impending future:

warning: 'constexpr' non-static member function will not be implicitly 'const' in C++1y; add 'const' to avoid a change in behavior [-Wconstexpr-not-const]

capnp compiles without other warnings, so I thought I'd keep the trend going.