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>

cache.h

@@ -1258,7 +1258,4 @@ extern struct startup_info *startup_info;
 /* builtin/merge.c */
 int checkout_fast_forward(const unsigned char *from, const unsigned char *to);
 
-/* in ctype.c, for kwset users */
-extern const char tolower_trans_tbl[256];
-
 #endif /* CACHE_H */

git-compat-util.h

@@ -457,6 +457,9 @@ static inline int has_extension(const char *filename, const char *ext)
 	return len > extlen && !memcmp(filename + len - extlen, ext, extlen);
 }
 
+/* in ctype.c, for kwset users */
+extern const char tolower_trans_tbl[256];
+
 /* Sane ctype - no locale, and works with signed chars */
 #undef isascii
 #undef isspace