src: fix solaris 10 build error

Stop gcc from getting confused, explicitly cast the return value from
getuid() and getgid() to uint32_t.  Fixes the following build error:

    ../src/node.cc: In function 'void node::GetUid(const
    v8::FunctionCallbackInfo<v8::Value>&)':
    ../src/node.cc:1552:37: error: call of overloaded 'Set(uid_t)' is
    ambiguous
    ../src/node.cc:1552:37: note: candidates are:
    ../deps/v8/include/v8.h:5939:6: note: void
    v8::ReturnValue<T>::Set(bool) [with T = v8::Value]
    ../deps/v8/include/v8.h:5909:6: note: void
    v8::ReturnValue<T>::Set(double) [with T = v8::Value]
    ../deps/v8/include/v8.h:5915:6: note: void
    v8::ReturnValue<T>::Set(int32_t) [with T = v8::Value, int32_t = int]
    ../deps/v8/include/v8.h:5926:6: note: void
    v8::ReturnValue<T>::Set(uint32_t) [with T = v8::Value, uint32_t =
    unsigned int]

Fixes #6182.