um: Give start_idle_thread() a return code

Fixes:
arch/um/os-Linux/skas/process.c:613:1: warning: control reaches end of
non-void function [-Wreturn-type]

longjmp() never returns but gcc still warns that the end of the function
can be reached.
Add a return code and debug aid to detect this impossible case.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/skas/process.c

@@ -610,6 +610,11 @@ int start_idle_thread(void *stack, jmp_buf *switch_buf)
 		fatal_sigsegv();
 	}
 	longjmp(*switch_buf, 1);
+
+	/* unreachable */
+	printk(UM_KERN_ERR "impossible long jump!");
+	fatal_sigsegv();
+	return 0;
 }
 
 void initial_thread_cb_skas(void (*proc)(void *), void *arg)