smixx stack : fix clang build error due to incorrect usage of default parameter value

Address build errors with clang introduced when smixx was added. e.g.

https://lhcb-nightlies.web.cern.ch/logs/build/nightly/lhcb-master-mr/280/x86_64-centos7-clang8-opt/Online/

Online/smixx/src/translator/stack.cxx:19:18: error: addition of default argument on redeclaration makes this constructor a default constructor
Stack::Stack(int size = 1000) : _maxLen(size), _top(EMPTY)
                 ^      ~~~~
Online/smixx/src/translator/stack.hxx:16:2: note: previous declaration is here
        Stack(int size);
        ^
2 warnings and 1 error generated.

note there are a number of additional warnings with clang, not addressed here. A number look potentially concerning so probably worth investigating.