whisper : fix grammar advance stack warning

[danbev]

This commit addresses a warnings that is present for Release builds:

[ 30%] Building CXX object src/CMakeFiles/whisper.dir/whisper.cpp.o
In file included from /usr/include/c++/13/bits/stl_tree.h:63,
                 from /usr/include/c++/13/map:62,
                 from /home/danbev/work/ai/whisper.cpp/src/whisper-arch.h:5,
                 from /home/danbev/work/ai/whisper.cpp/src/whisper.cpp:2:
In static member function ‘static void std::__copy_move<false, false, std::random_access_iterator_tag>::__assign_one(_Tp*, _Up*) [with _Tp = const whisper_grammar_element*; _Up = const whisper_grammar_element* const]’,
    inlined from ‘static _Up* std::__copy_move<_IsMove, true, std::random_access_iterator_tag>::__copy_m(_Tp*, _Tp*, _Up*) [with _Tp = const whisper_grammar_element* const; _Up = const whisper_grammar_element*; bool _IsMove = false]’ at /usr/include/c++/13/bits/stl_algobase.h:440:20,
    inlined from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = const whisper_grammar_element* const*; _OI = const whisper_grammar_element**]’ at /usr/include/c++/13/bits/stl_algobase.h:506:30,
    inlined from ‘_OI std::__copy_move_a1(_II, _II, _OI) [with bool _IsMove = false; _II = const whisper_grammar_element* const*; _OI = const whisper_grammar_element**]’ at /usr/include/c++/13/bits/stl_algobase.h:533:42,
...

This warning is caused by the fact that the stack vector is empty when it is passed to new_stacks.push_back(stack);.

The suggested fix is to use new_stacks.emplace_back(); instead of new_stacks.push_back(stack);.