modules/videoio/src/cap_ffmpeg_impl.hpp: fix build with gcc 4.8

modules/videoio/src/cap_ffmpeg_impl.hpp: fix build with gcc 4.8

Fix the following build failure with gcc 4.8:

In file included from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/cap_ffmpeg_impl.hpp:100:0,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/cap_ffmpeg.cpp:50:
/home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/cap_ffmpeg_hw.hpp: In constructor 'HWAccelIterator::HWAccelIterator(cv::VideoAccelerationType, bool, AVDictionary*)':
/home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/cap_ffmpeg_hw.hpp:939:23: error: use of deleted function 'std::basic_istringstream<char>& std::basic_istringstream<char>::operator=(const std::basic_istringstream<char>&)'
             s_stream_ = std::istringstream(accel_list);
                       ^
In file included from /home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/arm-none-linux-gnueabi/include/c++/4.8.3/complex:45:0,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/core/include/opencv2/core/cvstd.inl.hpp:47,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/core/include/opencv2/core.hpp:3306,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/include/opencv2/videoio.hpp:46,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/precomp.hpp:57,
                 from /home/buildroot/autobuild/instance-3/output-1/build/opencv4-4.5.4/modules/videoio/src/cap_ffmpeg.cpp:42:
/home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/arm-none-linux-gnueabi/include/c++/4.8.3/sstream:272:11: note: 'std::basic_istringstream<char>& std::basic_istringstream<char>::operator=(const std::basic_istringstream<char>&)' is implicitly deleted because the default definition would be ill-formed:
     class basic_istringstream : public basic_istream<_CharT, _Traits>
           ^
/home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/arm-none-linux-gnueabi/include/c++/4.8.3/sstream:272:11: error: use of deleted function 'std::basic_istream<char>& std::basic_istream<char>::operator=(const std::basic_istream<char>&)'

Fixes:
 - http://autobuild.buildroot.org/results/60f8846b435dafda0ced412d59ffe15bdff0810d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>