Fix {$LINKLIB} directive on Darwin

On Darwin, we were linking against SDL2, instead of SDL3.

Author: suve

units/SDL3.pas

@@ -51,7 +51,7 @@ interface
     {$IFDEF DARWIN}
       SDL_LibName = 'libSDL3.dylib';
       {$IFDEF FPC}
-        {$LINKLIB libSDL2}
+        {$LINKLIB libSDL3}
       {$ENDIF}
     {$ELSE}
       {$IFDEF FPC}