ctype.c: Fix a sparse warning

In particular, sparse complains as follows:

        SP ctype.c
    ctype.c:30:12: warning: symbol 'tolower_trans_tbl' was not declared.\
         Should it be static?

An appropriate extern declaration for the 'tolower_trans_tbl' symbol
is included in the "cache.h" header file. In order to suppress the
warning, therefore, we could replace the "git-compat-util.h" header
inclusion with "cache.h", since "cache.h" includes "git-compat-util.h"
in turn. Here, however, we choose to move the extern declaration for
'tolower_trans_tbl' into "git-compat-util.h", alongside the other
extern declaration from ctype.c for 'sane_ctype'.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>