um: Fix -Wmissing-prototypes warnings for (rt_)sigreturn

Use SYSCALL_DEFINE0 to define (rt_)sigreturn. This will address
below -Wmissing-prototypes warnings:

arch/x86/um/signal.c:453:6: warning: no previous prototype for ‘sys_sigreturn’ [-Wmissing-prototypes]
arch/x86/um/signal.c:560:6: warning: no previous prototype for ‘sys_rt_sigreturn’ [-Wmissing-prototypes]
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/signal.c

@@ -8,6 +8,7 @@
 #include <linux/personality.h>
 #include <linux/ptrace.h>
 #include <linux/kernel.h>
+#include <linux/syscalls.h>
 #include <asm/unistd.h>
 #include <linux/uaccess.h>
 #include <asm/ucontext.h>
@@ -449,7 +450,7 @@ int setup_signal_stack_si(unsigned long stack_top, struct ksignal *ksig,
 	return 0;
 }
 
-long sys_sigreturn(void)
+SYSCALL_DEFINE0(sigreturn)
 {
 	unsigned long sp = PT_REGS_SP(&current->thread.regs);
 	struct sigframe __user *frame = (struct sigframe __user *)(sp - 8);
@@ -556,7 +557,7 @@ int setup_signal_stack_si(unsigned long stack_top, struct ksignal *ksig,
 }
 #endif
 
-long sys_rt_sigreturn(void)
+SYSCALL_DEFINE0(rt_sigreturn)
 {
 	unsigned long sp = PT_REGS_SP(&current->thread.regs);
 	struct rt_sigframe __user *frame =