Fix: custom signal handler not working

kernel/lib/syscall.c

@@ -5,6 +5,7 @@
 #include <process.h>
 #include <mbox.h>
 #include <exec.h>
+#include <signal.h>
 
 void syscall_handler(trapframe_t *regs)
 {
@@ -110,8 +111,4 @@ void sys_kill(int32_t pid, int32_t SIGNAL)
 }
 
 void sys_sigreturn(void)
-{
-  DEBUG_SIGNAL("sigreturn");
-  thread_t *th = get_current();
-  user_exec(th->process->regs);
-}
+{ end_signal(); }