[PATCH] lib/string.c cleanup: restore useful memmove const

A couple of (char *) casts removed in a previous cleanup patch in
lib/string.c:memmove() were actually useful, as they suppressed a couple of
warnings:

	assignment discards qualifiers from pointer target type

Fix by declaring the local variable const in the first place, so casts
aren't needed to strip the const qualifier.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>