kwset: move translation table from ctype

This table was originally introduced to solely be used with kwset
machinery (0f871cf56e), so it would make sense for it to belong in
kwset.[ch] rather than ctype.c and git-compat-util.h. It is only used in
diffcore-pickaxe.c, which already includes kwset.h so no other headers
have to be modified.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

kwset.h

@@ -26,6 +26,8 @@
    The author may be reached (Email) at the address mike@ai.mit.edu,
    or (US mail) as Mike Haertel c/o Free Software Foundation. */
 
+extern const unsigned char tolower_trans_tbl[256];
+
 struct kwsmatch
 {
   int index;			/* Index number of matching keyword. */