dispatch: silence a warning on Windows (NFC)

clang objects to the return from function marked as noreturn.  Add in a
marker to indicate that the return path is dead.  This should be
equivalent to the existing code but silences the compiler warning.

Author: compnerd

src/queue.c

@@ -7813,6 +7813,7 @@ _dispatch_sig_thread(void *ctxt DISPATCH_UNUSED)
 	_dispatch_clear_stack(0);
 #if defined(_WIN32)
 	Sleep(INFINITE);
+	__assume(0);
 #else
 	_dispatch_sigsuspend();
 #endif