uml: fix warnings in kernel_execve

Fix the following warnings:

arch/um/kernel/syscall.c: In function 'kernel_execve':
arch/um/kernel/syscall.c:130: warning: passing argument 1 of 'um_execve' discards qualifiers from pointer target type
arch/um/kernel/syscall.c:130: warning: passing argument 2 of 'um_execve' discards qualifiers from pointer target type
arch/um/kernel/syscall.c:130: warning: passing argument 3 of 'um_execve' discards qualifiers from pointer target type

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>