src: fix compiler warning in smalloc.cc

Fix the following compiler warning by static_casting the enum values
to an uint32_t:

    ../src/smalloc.cc: In function 'void
    node::smalloc::Initialize(v8::Handle<v8::Object>,
                              v8::Handle<v8::Value>,
                              v8::Handle<v8::Context>)':
    ../src/smalloc.cc:601:203: warning: enumeral and non-enumeral type
    in conditional expression
        EXTERNAL_ARRAY_TYPES(V)

PR-URL: https://github.com/iojs/io.js/pull/1055


Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>