Fixed minor warning in Xcode.

Strange that VS only picked up the other instance of this issue, and neither gcc nor clang found either one.

src/emucore/System.cxx

@@ -39,7 +39,7 @@ System::System(const OSystem& osystem, M6502& m6502, M6532& m6532,
     mySystemInAutodetect(false)
 {
   // Re-initialize random generator
-  randGenerator().initSeed(myOSystem.getTicks());
+  randGenerator().initSeed(uInt32(myOSystem.getTicks()));
 
   // Initialize page access table
   PageAccess access(&myNullDevice, System::PA_READ);