Use noreturn attribute in Exit() declaration.

Use the gcc-specific noreturn attribute to advise the compiler that
Exit() doesn't return.  This is safe because we already use preprocessor
rules to hide __attribute__ from other compilers.

Suggested independently by Christophe Curis and Brad Jorsch.

src/funcs.h

@@ -35,7 +35,7 @@ void Shutdown(WShutdownMode mode);
 
 void RestoreDesktop(WScreen *scr);
 
-void Exit(int status);
+void Exit(int status) __attribute__((noreturn));
 
 void Restart(char *manager, Bool abortOnFailure);