testing/libretro-beetle-saturn: fix stdint missing

Code build error:

In file included from mednafen/Stream.cpp:20: mednafen/Stream.h:24:15: error: 'uint64_t' does not name a type 

24 | virtual uint64_t read(void \*data, uint64_t count) = 0; 

     | 

This issue has been fixed upstream：https://github.com/libretro/beetle-saturn-libretro/blob/master/mednafen/Stream.h