Add span to #include

[ryzendew]

Span was not included and gave me an error

/home/matt/rpcsx/rpcsx-gpu/main.cpp: In function ‘int main(int, const char**)’:
/home/matt/rpcsx/rpcsx-gpu/main.cpp:775:26: error: ‘span’ is not a member of ‘std’
  775 |     for (auto cmd : std::span(commandsBuffer, pulledCount)) {
      |                          ^~~~
gmake[2]: *** [rpcsx-gpu/CMakeFiles/rpcsx-gpu.dir/build.make:76: rpcsx-gpu/CMakeFiles/rpcsx-gpu.dir/main.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:394: rpcsx-gpu/CMakeFiles/rpcsx-gpu.dir/all] Error 2
gmake: *** [Makefile:136: all] Error 2

so doing #include <span> Fixes this error.