src: fix compiler warning in smalloc.cc

[bnoordhuis]

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)

R=@vkurchatkin

[vkurchatkin]

Thanks, landed in fb284e2