[S390] s390: fix build error (sys_execve)

fix this build error:
arch/s390/kernel/process.c:272: error: conflicting types for 'sys_execve'
arch/s390/kernel/entry.h:45: error: previous declaration of 'sys_execve' was here
make[1]: *** [arch/s390/kernel/process.o] Error 1
make: *** [arch/s390/kernel] Error 2

introduced by d7627467b7

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/entry.h

@@ -42,8 +42,8 @@ long sys_clone(unsigned long newsp, unsigned long clone_flags,
 	       int __user *parent_tidptr, int __user *child_tidptr);
 long sys_vfork(void);
 void execve_tail(void);
-long sys_execve(const char __user *name, char __user * __user *argv,
-		char __user * __user *envp);
+long sys_execve(const char __user *name, const char __user *const __user *argv,
+		const char __user *const __user *envp);
 long sys_sigsuspend(int history0, int history1, old_sigset_t mask);
 long sys_sigaction(int sig, const struct old_sigaction __user *act,
 		   struct old_sigaction __user *oact);