s390/signal: add sys_sigreturn and sys_rt_sigreturn declarations

Get rid of sparse warnings like this one:
arch/s390/kernel/signal.c:244:1:
  warning: symbol 'sys_sigreturn' was not declared. Should it be static?
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/kernel/entry.h

@@ -71,9 +71,11 @@ struct s390_mmap_arg_struct;
 struct fadvise64_64_args;
 struct old_sigaction;
 
+long sys_rt_sigreturn(void);
+long sys_sigreturn(void);
+
 long sys_s390_personality(unsigned int personality);
 long sys_s390_runtime_instr(int command, int signum);
-
 long sys_s390_pci_mmio_write(unsigned long, const void __user *, size_t);
 long sys_s390_pci_mmio_read(unsigned long, void __user *, size_t);
 #endif /* _ENTRY_H */