uml: fix compile warning

This fixes:
incompatible pointer type:  => 89
       arch/um/kernel/exec.c: warning: passing argument 2 of 'execve1' from
incompatible pointer type:  => 69, 85
       arch/um/kernel/exec.c: warning: passing argument 3 of 'execve1' from
incompatible pointer type:  => 69, 85

which was introduced by d7627467 ("Make do_execve() take a const
filename pointer")

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: David Howells <dhowells@redhat.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>