rcore_platform_sdl: Fix `GetTime()` resolution for sdl

`SDL_GetTicks()` only has millisecond resolution so switched to
`SLD_GetPerformanceCounter()` combined with
`SDL_GetPerformanceFrequency()` which should allow more granular timing

src/platforms/rcore_desktop_sdl.c

@@ -1276,8 +1276,7 @@ void SwapScreenBuffer(void)
 // Get elapsed time measure in seconds
 double GetTime(void)
 {
-    unsigned int ms = SDL_GetTicks();    // Elapsed time in milliseconds since SDL_Init()
-    double time = (double)ms/1000;
+    double time = (double)SDL_GetPerformanceCounter() / (double)SDL_GetPerformanceFrequency();
     return time;
 }