fix mingw x86_64 build

This fixes compilation on mingw x86_64 (was fine on i686).
Tested also on linux64.
This may be compiler-dependent and would require some testing on msvc 32/64 bits, linux32.

In file included from /home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/matrix.h:35:0,
                 from /home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/flann.hpp:41,
                 from /home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/flann.h:476,
                 from /home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/flann.cpp:31:
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h: In instantiation of ‘static void flann::serialization::access::serialize(Archive&, T&) [with Archive = flann::serialization::LoadArchive; T = long long unsigned int]’:
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:42:30:   required from ‘void flann::serialization::serialize(Archive&, T&) [with Archive = flann::serialization::LoadArchive; T = long long unsigned int]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:51:40:   required from ‘static void flann::serialization::Serializer<T>::load(InputArchive&, T&) [with InputArchive = flann::serialization::LoadArchive; T = long long unsigned int]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:293:61:   required from ‘Archive& flann::serialization::InputArchive<Archive>::operator&(T&) [with T = long long unsigned int; Archive = flann::serialization::LoadArchive]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:121:12:   required from ‘static void flann::serialization::Serializer<std::vector<T> >::load(InputArchive&, std::vector<T>&) [with InputArchive = flann::serialization::LoadArchive; T = unsigned int]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:293:61:   [ skipping 2 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:34:9:   required from ‘static void flann::serialization::access::serialize(Archive&, T&) [with Archive = flann::serialization::LoadArchive; T = flann::LshIndex<flann::KL_Divergence<int> >]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:42:30:   required from ‘void flann::serialization::serialize(Archive&, T&) [with Archive = flann::serialization::LoadArchive; T = flann::LshIndex<flann::KL_Divergence<int> >]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:51:40:   required from ‘static void flann::serialization::Serializer<T>::load(InputArchive&, T&) [with InputArchive = flann::serialization::LoadArchive; T = flann::LshIndex<flann::KL_Divergence<int> >]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:293:61:   required from ‘Archive& flann::serialization::InputArchive<Archive>::operator&(T&) [with T = flann::LshIndex<flann::KL_Divergence<int> >; Archive = flann::serialization::LoadArchive]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/algorithms/lsh_index.h:204:9:   required from ‘void flann::LshIndex<Distance>::loadIndex(FILE*) [with Distance = flann::KL_Divergence<int>; FILE = _iobuf]’
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/flann.cpp:893:1:   required from here
/home/xantares/projects/xantares-aur/mingw-w64-flann/src/flann-1.8.4-src/src/cpp/flann/util/serialization.h:34:9: error: request for member ‘serialize’ in ‘type’, which is of non-class type ‘long long unsigned int’
         type.serialize(ar);