SH: Add missing consts to sys_execve() declaration

Add missing consts to the sys_execve() declaration which result in the
following error:

arch/sh/kernel/process_32.c:303: error: conflicting types for 'sys_execve'
/warthog/nfs/linux-2.6-fscache/arch/sh/include/asm/syscalls_32.h:24: error: previous declaration of 'sys_execve' was here

Signed-off-by: David Howells <dhowells@redhat.com>