codec2: fix multiple definition error on linker

The following linker error occurs when you have more than one cpp file which
includes this header in your library.

"multiple definition of 'int const& android::C2Value::Primitive::ref<int>()
const'"
... and more

Use inline to avoid multiple definition.

Test: codec2 test
Change-Id: I3b9e026d70217fe82fd8eefee0bfd14406824219